IKELEY  LIBRAR 


RAILROAD  COMMISSION  OF  THE  STATE  OF  CALIFORNIA 


REPORT 


T 

BY   THE 


. 
Joint  Committee  on  Inductive 

Interference 


TO   THE 


Railroad  Commission  of  the  State 
of  California 


Approved  by  the  Railroad  Commission  on  July  30,  1914 


CALIFOBNIA 

STATE  PRINTING  OFFICE 
1914 


REPORT 


BY    THE 


Joint  Committee  on  Inductive 
Interference 


TO    THE 


Railroad  Commission  of  the  State 
of  California 


Presenting  the  results  accomplished  to  date  in  the  study  of  inductive  interference 

and  including  recommendations  for  rules  designed  to  prevent  or  mitigate 

inductive  interference  to  communication  circuits  by  power  circuits 


San  Francisco,  California,  July  7,  1914 


CALIFORNIA 
STATE  PRINTING  OFFICE 

1914 
12191 


CS/ 


LETTER  OF  APPROVAL. 

SAN  FRANCISCO,  July  30,  1914. 
Joint  Committee  on  Inductive  Interference: 

GENTLEMEN:  We  desire  to  acknowledge  receipt  of  yours  of  the  7th 
instant,  transmitting  the  report  of  the  Joint  Committee  on  Inductive 
Interference,  and  also  of  yours  of  the  23d  instant,  referring  in  greater 
detail  to  future  work  of  your  committee,  and  to  thank  you  for  the  same. 
.  The  Commission  realizes  the  arduous  labor  which  your  committee  and 
the  individual  members  thereof  have  performed  in  seeking  to  ascertain 
the  causes  of  inductive  interference  between  power  and  communication 
circuits,  and  to  prescribe  rules  and  regulations  for  preventing  or 
minimizing  such  interference,  and  extends  to  the  committee,  and  each 
member  thereof,  its  congratulations  on  the  results  accomplished  and 
its  thanks  for  the  scholarly,  scientific,  painstaking  manner  in  which 
the  work  has  been  performed. 

The  Commission  has  adopted  the  rules  as  proposed  by  your  com- 
mittee and  has  added  two  new  rules,  one  dealing  with  the  applicability 
of  the  rules  to  existing  and  future  construction  and  the  other  declar- 
ing the  principle  that  these  rules  shall  be  subject  to  the  laws  of  this 
State  and  the  orders  of  this  Commission,  now  or  hereafter  in  effect. 
The  Commission's  order  will  be  published  as  a  general  order. 

Your  report  will  be  printed  by  this  Commission  for  free  distribution. 
While  the  general  conclusions  will  be  given  to  the  press  in  the  usual 
course,  we  shall  be  glad  to  have  the  report  printed  in  full  in  the  Pro- 
ceedings of  the  American  Institute  of  Electrical  Engineers. 

The  Commission  hereby  requests  your  committee  to  continue  its 
work  along  the  lines  indicated  in  your  report  and  your  letter  of  the 
23d  instant,  and  authorizes  the  raising  of  the  necessary  funds  by 
assessment  as  heretofore,  with  the  understanding  that  the  Commission 
will  assign  one  of  its  stenographers  to  the  work  of  the  committee,  and 
that  our  engineering  department  will  at  all  times  co-operate  with  your 
committee. 

With  sincere  appreciation  for  the  work  hitherto  performed  and 
assurances  of  continued  interest  in  the  work  to  which  you  will  now 
devote  yourselves, 

We  remain,  respectfully, 

RAILROAD  COMMISSION, 

JOHN  M.  ESHLEMAN, 
H.  D.  LOVELAND, 
ALEX  GORDON, 
MAX  THELEN, 
E.  0.  EDGERTON, 

Commissioners. 


LETTER  OF  TRANSMITTAL. 

SAN  FRANCISCO,  July  7,  1914. 
To  the  Railroad  Commission  of  the  State  of  California, 

San  Francisco,  California: 

GENTLEMEN  :  The  Joint  Committee  on  Inductive  Interference  sub- 
mits herewith  a  report  based  on  its  work  to  date,  containing  provisional 
rules  which  tend  to  improve  conditions  in  respect  to  inductive  inter- 
ference. The  investigation  undertaken  by  the  committee  has  not  been 
completed,  but  the  results  already  obtained  serve  to  point  out  a  number 
of  requirements  and  precautionary  measures  which  should  be  complied 
with  in  future  work.  These  have  been  embodied  in  the  rules  presented 
herewith,  and  it  is  the  recommendation  of  the  committee  that  these 
rules  he  made  effective  immediately  without  waiting  for  the  completion 
of  the  investigation. 

The  committee  desires  to  explain,  in  respect  to  certain  of  the  rules, 
that  while  the  general  character  of  their  essential  provisions  is  well 
understood,  the  information  available  at  present  is  not  sufficiently  com- 
plete to  make  it  possible  to  set  definite  quantitative  limits  and  to  make 
all  the  rules  explicit,  such  as  they  should  be  in  order  to  afford  the  maxi- 
mum reduction  of  inductive  interference  consistent  with  the  burdens 
imposed  by  the  rules.  In  a  few  instances,  rules  have  been  drawn  with 
definite  limits  which  have  been  set  somewhat  arbitrarily,  in  accordance 
with  the  committee's  best  judgment.  Therefore,  the  rules  are  not  put 
forth  as  being  complete  or  final,  but  must  be  regarded  as  provisional 
and  subject  to  such  change  as  the  results  of  further  investigation  and 
experience  may  determine.  They  are,  however,  recommended  unani- 
mously by  the  committee  as  the  best  which  can  be  formulated  at  this 
time,  and  thus  having  the  support  of  all  the  interests  represented  on 
the  committee,  it  is  hoped  that  the  rules  will  appeal  to  the  Commission 
as  being  reasonable  and  proper. 

The  report  also  outlines  other  experimental  work,  some  of  which  is 
now  in  progress,  which  the  committee  considers  essential  in  order  that 
additional  information  may  be  acquired  for  amplifying  and  revising 
these  rules  to  make  them  more  definite  and  complete. 
Respectfully  submitted, 

(Signed)         RICHARD  SACHSE, 
A.  H.  BABCOCK, 
R.  W.  GRAY, 
F.  EMERSON  HOAR, 
J.  E.  WOODBRIDGE, 
J.  P.  JOLLYMAN, 
P.  M.  DOWNING, 
H.  A.  BARRE, 
C.  H.  TEMPLE, 
A.  H.  GRISWOLD, 
R.  W.  MASTICK, 
V.  V.  STEVENSON, 
J.  A.  KOONTZ, 
A.  L.  WILSON, 

Joint  Committee  on  Inductive  Interference. 
Honorary  Members : 

HOWARD  S.  WARREN. 
JAMES  T.  SHAW. 


REPORT 

BY   THE 

Joint  Committee  on  Inductive  Interference  to 

the  Railroad  Commission  of  the  State  of 

California. 

SCOPE. 

This  report  presents  briefly  an  account  of  the  formation  of  this 
committee,  its  activities  and  results  accomplished  to  date,  and  recom- 
mendations for  such  rulings  by  the  Railroad  Commission  of  the  State 
of  California  as  the  committee  believes  are  justified  at  this  time; 
together  with  a  technical  discussion  in  explanation  of  the  results  and 
recommendations. 

HISTORICAL. 

The  formation  of  the  Joint  Committee  on  Inductive  Interference  was 
the  outgrowth  of  certain  differences  involving  power,  communication, 
and  railroad  interests  which  were  brought  to  the  attention  of  the  Rail- 
road Commission  of  California.  As  an  alternative  to  contesting  the 
issue  at  that  time  it  wras  agreed  by  the  power  and  communication  com- 
panies, with  the  approval  of  the  Commission,  that  a  joint  investigation 
should  be  made  to  obtain  certain  information  essential  to  a  proper 
solution  of  the  difficulties.  The  Commission  desired  that  the  matter  be 
thoroughly  investigated  before  passing  upon  the  general  principles 
involved  in  these  difficulties.  To  this  end  a  general  conference  was 
called  to  select  representatives  to  form  a  "Joint  Committee"  empowered 
to  conduct  tests,  experiments,  and  investigations,  the  results  of  which 
would  serve  as  a  basis  of  recommendations  for  rules  and  regulations  to 
be  issued  by  the  Commission,  tending  to  minimize  inductive  interference 
and  physical  hazard  arising  from  parallelism  of  different  classes  of 
circuits.  This  conference  was  held  December  16,  1912.  As  a  result 
the  Joint  Committee  on  Inductive  Interference,  representing  the  Rail- 
road Commission  and  railroad,  power,  and  .communication  interests  of 
the  State,  was  organized  and  authorized  by  the  Railroad  Commission  of 
California  to  conduct  the  desired  investigation. 

The  personnel  of  the  committee  selected  is  given  below. 
Representing  Railroad  Commission: 

Mr.  R.  A.  Thompson,  Chief  Engineer. 

Mr.  A.  R.  Kelley,  Assistant  Engineer. 

Mr.  James  T.  Shaw,  Assistant  Rate  Expert. 

Mr.  F.  Emerson  Hoar,  Assistant  Rate  Expert. 
Representing  Railroad  Interests: 

Mr.   A.    H.    Babcock,    Consulting   Electrical   Engineer,    Southern 

Pacific  Company. 
Representing  Telephone  and  Telegraph  Interests: 

Mr.  A.  H.  Griswold,  Plant  Engineer,  The  Pacific  Telephone  and 
Telegraph  Company. 

Mr.  R.  W.  Gray,  Division  Superintendent,  Western  Union  Tele- 
graph Company. 


6  REPORT    ON    INDUCTIVE    INTERFERENCE. 

Mr.  C.  H.  Temple,  General  Manager,  United  States  Long  Distance 
Telephone  Company. 

Mr.  L.  M.  Ellis,  General  Manager,  Union  Home  Telephone  Com- 
pany. 

Representing  Power  Interests: 

Mr.  H.  A.  Barre,  Electrical  Engineer,  Pacific  Light  and  Power 

Corporation. 

Mr.  Louis  Elliott,  Engineer,  Great  Western  Power  Company. 
Mr.  P.  M.  Downing,  Engineer,  Pacific  Gas  and  Electric  Company. 
Mr.  J.  E.  Woodbridge,  Chief  Engineer,  Sierra  and  San  Francisco 

Power  Company. 

The  organization  and  personnel  of  the  Joint  Committee  on  Inductive 
Interference  were  approved  by  the  Railroad  Commission  on  January  6, 
1913,  and  the  committee  thereupon  proceeded  with  the  necessary  tests 
and  investigations. 

For  the  more  efficient  conduct  of  its  work  the  Joint  Committee  was 
divided  into  several  smaller  subcommittees,  each  assigned  to  and  respon- 
sible for  certain  branches  of  the  investigation.  The  present  organiza- 
tion of  the  Joint  Committee  is  given  on  a  chart  presented  as  Ap- 
pendix VI. 

Since  the  formation  of  the  committee,  through  additions,  resignation 
or  death,  the  personnel  of  the  committee  has  changed  as  follows : 

Mr.  Louis  Elliott  resigned  and  Mr.  J.  A.  Koontz,  Engineer  of  the 
Great  Western  Power  Company,  was  appointed  in  his  place. 

Mr.  V.  V.  Stevenson,  Electrical  Engineer  of  the  Postal  Telegraph 
Cable  Company,  and  Mr.  L.  N.  Peart,  General  Superintendent  of  the 
San  Joaquin  Light  and  Power  Company,  were  added  to  the  original 
membership  by  action  of  the  committee. 

Mr.  R.  A.  Thompson,  Chairman  of  the  Joint  Committee,  resigned. 
Mr.  W.  C.  Earle,  his  successor  as  Chief  Engineer  of  the  Commission, 
was  elected  to  membership  and  chairmanship.  Subsequently  Mr.  Earle 
resigned  and  Mr.  Richard  Sachse,  Acting  Chief  Engineer  of  the  Rail- 
road Commission,  was  elected  to  membership  and  chairmanship. 

Mr.  L.  M.  Ellis  resigned  and  Mr.  R.  W.  Mastick,  Transmission  and 
Protection  Engineer  of  The  Pacific  Telephone  and  Telegraph  Company, 
was  elected  to  membership. 

Mr.  H.  S.  Warren,  Electrical  Engineer  of  the  American  Telephone 
and  Telegraph  Company,  was  elected  to  honorary  membership. 

Mr.  James  T.  Shaw,  Secretary  of  the  Joint  Committee,  resigned.  Mr. 
A.  R.  Kelley  was  elected  to  the  office  of  secretary.  The  vacancy  in 
membership  created  by  the  resignation  of  Mr.  Shaw  was  later  filled  by 
the  election  of  Mr.  A.  L.  Wilson,  Assistant  Rate  Expert  of  the  Railroad 
Commission.  Mr.  James  T.  Shaw  was  elected  to  honorary  membership. 

The  death  of  Mr.  L.  N.  Peart  created  a  vacancy  in  membership  which 
was  filled  by  the  election  of  Mr.  J.  P.  Jollyman,  Engineer  of  Electrical 
Construction  of  the  Pacific  Gas  and  Electric  Company. 

Mr.  A.  R.  Kelley  resigned.  To  date  the  vacancy  created  by  Mr. 
Kelley 's  resignation  has  not  been  filled  nor  has  a  permanent  secretary 
been  elected. 

Early  in  its  work  the  Joint  Committee  established  a  field  engineering 
staff,  reporting  to  the  Subcommittee  on  Tests,  to  conduct  the  necessary 
tests  and  investigations.  This  field  staff  was  composed  of  engineers 


REPORT    ON    INDUCTIVE    INTERFERENCE.  7 

in  the  employ  of  The  Pacific  Telephone  and  Telegraph  Company  and 
the  American  Telephone  and  Telegraph  Company,  and  was  later  aug- 
mented by  the  addition  of  two  engineers  and  a  stenographer  engaged 
by  the  Joint  Committee. 

Previous  to  the  formation  of  this  committee  in  December,  1912,  The 
Pacific  Telephone  and  Telegraph  Company  had  started  an  investigation 
(•-•f  inductive  interference  between  the  lines  of  the  Coast  Counties  Gas 
and  Electric  Company  and  the  lines  of  the  telephone  company  in  the 
neighborhood  of  Morgan  Hill  in  Santa  Clara  County.  This  investiga- 
tion was  completed  by  the  Joint  Committee  and  its  results  have  been 
considered  in  connection  with  other  work  carried  out  by  the  Joint 
Committee. 

In  January,  1913,  the  Joint  Committee  established  its  field  staff  at 
Salinas,  to  investigate  parallels  on  the  lines  of  the  Sierra  and  San 
Francisco  Power  Company  north  of  Salinas  and  on  the  line  of  the 
Coast  Valleys  Gas  and  Electric  Company  south  of  Salinas,  both  of  these 
power  lines  being  parallel  with  the  lines  of  The  Pacific  Telephone  and 
Telegraph  Company,  the  Western  Union  Telegraph  Company  and  the 
Southern  Pacific  Company's  signalling  system.  The  investigation  at 
Salinas  continued  from  January,  1913,  until  July,  1913. 

The  specific  work  undertaken  at  Salinas  was :  a  determination  of  the 
magnitude  and  characteristics  of  the  induction  produced  in  the  com- 
munication circuits,  the  factors  in  the  power  circuits  causing  this  induc- 
tion, the  quantitative  relationship  of  cause  and  effect,  and  a  comparison 
of  the  effects  on  the  parallels  north  of  Salinas  with  the  neutral  of  the 
power  circuit  alternatively  grounded  beyond  one  end  of  the  parallel 
and  beyond  both  ends  of  the  parallel. 

In  July,  1913,  the  field  headquarters  were  moved  to  Santa  Cruz.  At 
this  point  the  committee  desired  to  test  the  relative  merits  of  various 
schemes  of  transpositions  for  both  power  and  telephone  circuits,  and 
to  complete  the  investigation  begun  at  Morgan  Hill  on  the  system  of 
the  Coast  Counties  Gas  and  Electric  Company,  which  system  is  of  a 
different  character  from  that  studied  at  Salinas.  A  mathematical  study 
of  transpositions  in  general,  and  particularly  of  those  for  the  parallel 
between  Santa  Cruz  and  Watsonville,  has  been  completed.  The  experi- 
mental study  of  these  schemes  of  transpositions  has  not  yet  been 
completed. 

Owing  to  the  peculiar  nature  of  the  experimental  work  and  the 
refinements  required,  suitable  apparatus  was  not  easily  obtainable  and  in 
many  instances  it  was  necessary  to  design  and  develop  special  apparatus 
for  certain  of  the  tests.  A  considerable  amount  of  time  has  necessarily 
been  spent  at  all  points  of  the  tests  in  choosing  from  the  almost  innum- 
erable things  which  could  be  investigated  with  profit,  those  of  greatest 
value  which  could  be  carried  out  with  the  means  at  hand. 

In  the  course  of  its  investigations  the  committee  has  prepared  a  series 
of  fifty  technical  reports  which  present  and  discuss  in  detail  the  various 
features  of  the  work,  the  methods  and  apparatus  employed  and  the 
results  accomplished.  These  reports,  which  are  on  file  at  the  committee 
headquarters  in  the  offices  of  the  Railroad  Commission  of  California, 
are  listed  in  Appendix  V. 


2—12191 


8  REPORT    ON    INDUCTIVE   INTERFERENCE. 

RESULTS  ACCOMPLISHED. 

The  following  paragraphs  summarize  very  briefly  the  principal 
results  accomplished  to  date.  These  statements  of  results  are  accom- 
panied by  brief  explanatory  comment  upon  the  conclusions  reached. 
The  reasons  for  and  explanations  of  these  conclusions  are  given  in  more 
detail  in  the  appendices,  to  which  reference  is  made. 

1.  Interference  to  telephone  circuits  under  normal  operating  condi- 
tions of  power  circuits  arises  almost  wholly  from  the  harmonic  voltages 
and  currents  of  the  power  system.     (See  Appendix  I.) 

This  is  due  chiefly  to  the  fact  that  the  frequencies  of  the  harmonics 
generally  present  in  the  voltages  and  currents  of  power  systems  cover 
a  considerable  portion  of  the  range  of  the  voice  frequencies,  particularly 
those  frequencies  at  which  telephone  instruments  and  the  human  ear  are 
of  maximum  sensibility.  Extraneous  currents  of  frequencies  approach- 
ing the  average  voice  frequency  have  a  more  injurious  effect  upon  tele- 
phone conversation  than  currents  of  lower  frequencies. 

2.  The  effect  of  induction  of  the  fundamental  frequency  on  telephone 
circuits  is  comparatively  unimportant  unless  it  is  of  magnitude  sufficient 
to  constitute  a  physical  hazard.     (See  Appendix  I.) 

This  is  due  to  the  fact  that  the  fundamental  approaches  the  lower 
limit  of  audible  frequencies,  at  which  the  telephone  and  the  human  ear 
are  not  efficiently  responsive. 

3.  Interference  to  telegraph  and  other  signalling  circuits  is  due  prin- 
cipally to  the  fundamental  and  lower  harmonics.     (See  Appendix  I.) 

Telegraph  receiving  instruments  are  relatively  insensitive,  as  com- 
pared with  the  telephone,  to  the  higher  harmonics,  but  are  sensitive 
to  disturbances  of  lower  frequencies,  such  as  the  fundamental  and 
lower  harmonics  which  more  nearly  approach  the  normal  operating 
frequency  of  such  circuits. 

4.  The  power  circuit  currents  and  voltages  may  be  divided  into  two 
factors:    balanced  and  residual,   of  which,  for  equal  magnitude,   the 
latter  in  general  produce   the  greater  inductive  interference.      (See 
Appendix  II.) 

Residual  currents  and  voltages  act  inductively  in  a  similar  manner  to 
single-phase  currents  and  voltages  acting  in  a  circuit  composed  of  the 
line  conductors  in  parallel  with  earth  return,  which  is  a  condition 
favorable  to  very  large  induction.  Moreover,  such  a  circuit  which  in- 
cludes the  earth  as  one  side  can  not  be  transposed.  Transpositions  in 
the  power  circuit  can  not  reduce  the  inductive  effect  of  residuals  except 
as  they  reduce  the  magnitudes  of  the  residuals  themselves,  which 
they  do  in  some  cases.  The  inductive  interference  arising  from  such 
currents  and  voltages  can  be  reduced  only  in  the  case  of  metallic  cir- 
cuits, such  as  telephone  circuits,  by  transposing  these  circuits.  It  is, 
therefore,  important  that  the  telephone  circuits  be  transposed  at  fre- 
quent intervals  throughout  parallels  and  carefully  balanced  throughout 
their  entire  length,  and  that  the  residual  currents  and  voltages  be  kept 
sufficiently  small  to  give  negligible  induction  in  telephone  circuits  so 
arranged. 

5.  Inductive  interference  to  communication  circuits,  arising  from  the 
balanced  voltages  and  currents  can  in  a  large  measure  be  prevented  by 
means  of  an  adequate  system  of  transpositions  applied  to  both  power 
and   communication   circuits    (assuming   the   latter  are   metallic)    and 
located  with  due  regard  to  each  other. 


REPORT    ON   INDUCTIVE   INTERFERENCE.  9 

This  is  accomplished  partly  by  creating  mutually  neutralizing  induc- 
tive effects  in  neighboring  lengths  of  each  side  of  the  communication 
circuit  or  circuits  by  transposing  the  power  circuit,  and  partly  by 
equalizing  the  inductive  effects  on  the  two  sides  of  the  communication 
circuit  or  circuits  by  exposing  each  side  equally  to  the  influence  of  the 
power  circuit  by  transposing  the  communication  circuit. 

6.  Abnormal  conditions  and  at  times  switching  operations  produce 
transient  disturbances  of  a  very  severe  character. 

This  is  due  to  the  fact  that  abnormal  conditions  almost  invariably 
give  rise  to  residuals  of  large  magnitude,  often  including  high  har- 
monics. Abnormal  occurrences  incident  to  electrical  power  transmis- 
sion do  not  give  warning  of  their  occurrence,  and  since  they  can  not  be 
produced  artificially  on  transmission  systems  without  subjecting  the 
apparatus  to  great  risk  or  danger,  it  has  been  deemed  unwise  to  attempt 
any  experimental  tests  of  these  effects.  This  conclusion  is  therefore 
drawn  from  general  experience  and  data  of  actual  occurrences  collected 
by  the  committee. 

RULES   RECOMMENDED   BY  THE  COMMITTEE. 

The  following  are  the  rules  which  the  committee,  as  the  result  of  its 
study  to  date,  recommends  be  issued  at  this  time  to  govern  the  future 
construction  and  operation  of  power  and  communication  circuits  which 
are  or  are  proposed  to  be  so  located  as  to  create  a  parallel,  as  herein- 
after defined : 

OUTLINE  OF  RULES. 
DEFINITIONS. 

a.  Power  Circuit. 

b.  Communication  Circuit. 

c.  Telephone  Circuit. 

d.  Line. 

e.  Parallel  or  Parallelism. 

f.  Residual  Current. 

g.  Residual  Voltage. 
h.     Transposition. 

I.  AVOIDANCE  OF  PARALLELISM. 
II.  CONDITIONS  UNDER  WHICH  PARALLELISM  WILL  BE  PERMITTED. 

a.  Minimum  Horizontal  Separation. 

b.  Balance  of  Power  System. 

c.  Limitation  of  Residual  Currents  and  Voltages. 

d.  Transpositions  Inside  Limits  of  Parallel. 

e.  Transpositions   Outside  Limits   of  Parallel. 

f.  Uniformity  of  Parallel. 

g.  Transformer  Connections. 
h.     Switch  Equipment. 

i.     Switching. 

j.     Use  of  Air  Switches. 

k.     Abnormal  Conditions. 

1.     Devices    for  Indicating   Abnormal   Conditions   on    Systems   Isolated   from 

Ground. 

m.     Procedure  Under  Abnormal  Conditions. 
n.     Ammeters  in  Neutral  Ground  Connections. 
o.     Charging  Electrolytic  Lightning  Arresters. 
p.     Wave  Form  of  Rotating  Machines. 
q.     Exciting  Current  of  Transformers. 

III.  PROVISIONS  APPLYING  TO  EXISTING  PARALLELS. 

IV.  WAIVER  OF  CONDITIONS  BY  COMMUNICATION  COMPANY. 
V.  PARALLELISM  WITH  ALTERNATING  CURRENT  RAILWAYS. 


10  REPORT  ON  INDUCTIVE  INTERFERENCE. 

DEFINITIONS. 

The  following  definitions  are  given  of  certain  technical  terms  em- 
ployed herein : 

a.  Power  Circuit.     The  term  "power  circuit"  includes  any  over- 

head constant  potential  alternating  current  power  transmis- 
sion or  distribution  circuit  or  electrically  connected  network 
which  has  a  voltage  of  five  thousand  volts  or  more  between 
any  two  conductors,  or  of  three  thousand  volts  or  more  be- 
tween any  conductor  and  ground. 

b.  Communication    Circuit.     The   term   "communication   circuit" 

includes  any  overhead,  open  wire  telephone,  telegraph,  or 
signalling  circuit  which  is  used  in  the  service  of  the  public. 

c.  Telephone  Circuit.     The  term  "telephone  circuit"  includes  any 

inter-exchange  metallic  telephone  circuit,  and  therefore  ex- 
cludes subscribers'  circuits.  This  term  also  includes  any 
metallic  telephone  circuit  operated  by  any  railroad  or  other 
company  for  dispatching  purposes  or  for  public  use  between 
separate  communities. 

d.  Line.     The  term  "line"  means  any  circuit  or  aggregation  of 

circuits  carried  on  poles  or  towers. 

e.  Parallel  or  Parallelism.     The  terms  ' '  parallel "  or  "  parallelism ' ' 

refer  to  cases  where  a  power  line  and  a  communication  line 
follow  substantially  the  same  course,  or  are  otherwise  in  prox- 
imity for  a  sufficient  distance,  so  that  the  power  circuit  is 
liable  to  create  inductive  interference  in  the  communication 
circuits. 

f.  Residual  Current.    'The  term  "residual  current"   denotes  the 

vector  sum  of  the  currents  in  the  several  conductors  of  a  power 
circuit. 

g.  Residual   Voltage.     The   term   "residual   voltage"   denotes   the 

vector  sum  of  the  voltages  to  ground  of  the  several  conductors 
of  a  power  circuit. 

h.  Transposition.     The   term   "transposition"   denotes   the   inter- 
change of  position  of  the  several  conductors  of  a  circuit. 

I.  AVOIDANCE  OF  PARALLELISM. 

Every  reasonable  effort  shall  be  made  to  avoid  new  parallelism.  The 
party  proposing  to  build  a  new  communication'  or  power  line,  which 
will  create  a  parallel,  or  generally  to  reconstruct  an  existing  line  in- 
volved in  a  parallel  shall  give  due  notice  (at  least  thirty  days  wherever 
possible)  of  its  intention  to  the  other  party,  including  detailed  informa- 
tion as  to  the  location  and  character  of  the  proposed  line.  If  a  plan 
can  be  devised  and  agreed  upon  by  the  two  parties  for  maintaining  an 
adequate  separation  between  the  two  classes  of  lines  so  as  to  avoid  inter- 
ference, this  shall  be  done.  In  case  it  is  impracticable  to  secure 
adequate  separation  between  a  power  line  and  a  communication  line, 
parallelism  will  be  permitted,  subject  to  the  conditions  set  forth  in  II. 

II.  CONDITIONS  UNDER  WHICH  PARALLELISM  WILL  BE  PERMITTED. 

a.  Minimum  Horizontal  Separation.  The  minimum  horizontal  sep- 
aration between  the  power  line  and  communication  line  shall  be  equal  to 


REPORT    ON    INDUCTIVE    INTERFERENCE.  11 

the  height  of  the  taller  line.  The  only  exceptions  to  this  provision  are 
angle  crossings  and  other  unavoidable  cases  of  close  proximity,  and  in 
all  such  cases  the  power  line  shall  be  kept  above  the  communication 
line  and  constructed  in  conformity  with  the  National  Electric  Light 
Association's  specifications  for  overhead  crossings  or  other  approved 
equivalent  which  may  be  agreed  to  by  both  companies. 

b.  Balance   of  Power  System.     The  power  company  shall  exercise 
due  diligence    to  keep  the  currents  in,  and  the  voltages  to  ground  of, 
the  conductors  of  any  power  circuit  involved  in  a  parallel  as  closely 
balanced   as   practicable.     In   all   cases  where   telephone   circuits  are 
involved,   special   consideration   shall  be   given  to  the  prevention   or 
elimination  of  harmonics  in  the  residual  current  and  in  the  residual 
voltage. 

c.  Limitation  of  Residual  Currents  and   Voltages.     Pending  addi- 
tional rules  on  specific  means  other  than  those  given  herein,  the  parties 
concerned  shall  endeavor  to  agree  upon  the  means  to  be  employed  for 
the  prevention  or  limitation  of  residual  currents  and  voltages,  and  in 
the  event  of  disagreement  the  matter  shall  be  referred  to  the  Railroad 
Commission  of  the  State  of  California. 

d.  Transpositions  Inside  Limits  of  Parallel.     An  adequate  system  of 
transpositions  shall  be  installed  in  the  power  circuit  (or  circuits),  and 
in    the    communication    circuit    (or    circuits)    provided   the    latter   is 
metallic.     When  both  circuits  are  transposed  the  transpositions  in  both 
the  communication  and  power  circuits  shall  be  located  with  due  regard 
to  each  other. 

Every  reasonable  effort  shall  be  made  by  both  parties  concerned  to 
fix  the  limits  of  the  parallel  and  the  location  of  crossings,  branch  lines, 
and  connected  apparatus  so  as  to  facilitate  the  application  of  an  effective 
transposition  scheme. 

In  the  case  of  a  parallel  between  a  power  line  and  a  telephone  line 
the  company  owning  or  operating  the  telephone  line  involved  shall 
have  the  right  to  specify  the  number,  type  (in  respect  to  electrical  char- 
acteristics) and  location  of  the  transpositions  in  the  power  circuit, 
subject  to  the  following  limitations : 

1.  For  power  circuits  of  50,000  volts  or  over  the  average  distance 
between  successive  transpositions  shall  not  be  required  to  be  less 
than  one  mile  and  the  minimum  distance  between  any  two  succes- 
sive transpositions  shall  not  be  required  to  be  less  than  two  thirds 
of  a  mile. 

2.  For  power  circuits  of  less  than  50,000  volts  the  distance  be- 
tween successive  transpositions  shall  not  be  required  to  be  less  than 
one  sixth  of  a  mile. 

The  transposition  system  of  the  telephone  circuits  shall  be  modified 
where  necessary  in  order  that  the  power  and  telephone  circuits  shall 
be,  as  nearly  as  practicable,  mutually  non-inductive. 

For  short  parallels  less  than  six  miles  in  length  (or  short  sections 
of  longer  parallels  which  have  to  be  treated  independently  because  of 
abrupt  change  in  conditions)  with  power  circuits  of  50,000  volts  or  over, 
where  it  is  impracticable  to  obtain  an  adequate  balance  by  the  location 
of  transpositions  in  accordance  with  the  limit  specified  above,  the  com- 
pany owning  or  operating  the  telephone  line  involved  shall  have  the 


12  REPORT    ON    INDUCTIVE   INTERFERENCE. 

right  to  specify  the  number,  type  and  location  of  transpositions,  pro- 
vided the  distance  specified  between  successive  transpositions  is  not 
less  than  one  half  mile. 

When  necessary  (due  to  variations  in  lengths  of  telephone  transposi- 
tion sections)  in  order  to  secure  an  adequate  balance,  a  reduction  of  10 
per  cent  in  the  limiting  distances  between  successive  power  circuit 
transpositions  as  given  above  shall  be  allowed. 

In  the  case  of  a  parallel  between  a  power  line  and  a  telegraph  line 
or  other  grounded  communication  circuit,  the  location  of  the  transposi- 
tions in  the  power  line  shall  be  with  due  regard  to  the  limits  of  the 
parallel  in  order  to  form  as  nearly  as  practicable  a  balanced  system. 
The  location  and  type  of  such  transpositions  shall  be  as  specified  by 
the  communication  company,  subject  to  the  condition  that  the  trans- 
po-itions  in  the  power  circuit  may  not  be  required  to  be  less  than  one 
mile  apart. 

In  no  case  shall  the  power  company  be  required  to  relocate  poles  or 
towers  far  the  transpositions. 

The  parties  concerned  in  any  proposed  parallel  shall  endeavor  to 
agree  upon  a  transposition  scheme  for  such  parallel  in  accordance 
with  the  above.  In  the  event  of  a  disagreement,  the  matter  shall  be 
referred  to  the  Railroad  Commis-ion  of  the  State  of  California. 

e.  Transpositions  Outside  Limits  of  Parallel.     In  addition  to  trans- 
positions within  the  limits  of  a  parallel,  as  provided  in  "d"  hereof, 
each  new  power  circuit  isolated  from  ground    (or  extension  of  such 
existing  circuit)    which   is   constructed  subsequent  to  the   date  when 
these  rules  become  effective,  shall  be  transposed  throughout  its  entire 
length  in  such  manner  as  to  balance  the  electrostatic  capacities  to  earth 
of  its  several  conductors,  so  as  to  avoid  inequalities  among  the  voltages 
to  earth  of  the  several  conductors,  which  would  create  inductive  inter- 
ference.    Such  transpositions  shall  not  be  more  than  eight  miles  apart, 
provided,  however,  that  circuits  less  than  three  miles  in  length  are  not 
required  to  be  transposed  until  they  are  extended  to  a  greater  length; 
except  that  extensions  or  spurs  from  existing  lines,   the  electrostatic 
capacities  to  earth  of  whose  conductors  are  balanced,  shall  be  so  con- 
structed as  not  to  change  materially  the  balance,  of  the  existing  lines  to 
which  they  are  connected. 

f.  Uniformity  of  Parallel.     To  facilitate  the  application  of  effective 
transpositions,  both  parties  shall  endeavor  to  maintain  uniform  separa- 
tion, uniform  arrangement  of  conductors  and  uniform  relative  location 
of  the  two  classes  of  circuits  within  the  limits  of  a  parallel.     However, 
when  it  is  feasible  to  secure  a  substantial  increase  of  separation  between 
the  two  lines  for  a  considerable  portion  of  a  parallel  this  shall  be  done, 
as  such  an  increase  of  separation  is  of  more  benefit  than  uniformity. 

g.  Transformer  Connections.     (1)   On  any  power  circuit  involved  in 
a  parallel,  no  grounded  single-phase,  or  grounded  open-star  transformer 
connections  shall  be  employed. 

NOTE. — This  does  not  apply  to  railroads  operating  alternating  current  trolleys  with 
ground  return  which  are  covered  by  V. 

(2)  On  a  power  circuit  involved  in  a  parallel  no  star-connected  trans- 
formers or  auto-transformers  with  grounded  neutral  shall  be  employed, 
unless  delta-connected  secondary  or  tertiary  windings  or  other  equiva- 


REPORT    ON    INDUCTIVE   INTERFERENCE.  13 

lent  means  are  used  of  suppressing  the  third  harmonic  components  of 
the  residual  voltages  and  currents  introduced  by  the  transformers. 

(3)  Where  single-phase  loads  are  connected  to  a  polyphase  power 
circuit  involved  in  a  parallel  the  power  company  shall  endeavor  to 
arrange  successive  connections  of  this  type  so  as  to  equalize  the  loads 
upon  the  several  phases. 

(4)  On  a  three-phase  circuit  involved  in  a  parallel,  the  power  company 
shall  use,  wherever  practicable,  a  closed-delta  connection  in  preference 
to  an  open-delta  connection,  and  where  the  latter  is  employed  an  effort 
shall  be  made  to  distribute  such  connections  equally  upon  the  several 
phases. 

h.  Switch  Equipment.  A  power  circuit  involved  in  a  parallel  shall 
be  equipped,  between  the  source  of  supply  and -the  parallel,  with  oil 
switches,  all  poles  of  which  shall  be  mechanically  interconnected  for 
simultaneous  action.  With  the  exception  of  stations  where  an  operator 
is  constantly  on  duty,  these  switches  shall  be  rendered  automatic  for 
short-circuits,  grounds,  and  abnormal  neutral  currents. 

i.  Switching.  All  switching  on  all  parts  of  a  system  connected  to 
a  circuit  involved  in  a  parallel,  which  causes  harmful  transient  disturb- 
ances in  communication  circuits,  shall  be  done  by  means  of  oil  switches, 
all  poles  of  which  are  mechanically  interconnected  for  simultaneous 
operation. 

j.  Use  of  Air  Switches.  The  use  of  air  switches,  on  a  power  circuit 
involved  in  a  parallel,  is  prohibited  except  for  purposes  of  isolating 
sections  of  dead  line,  or  for  disconnecting  transformers  under  no  load. 
This  applies  to  the  entire  power  system,  any  circuit  of  which  is  involved 
in  a  parallel,  unless  such  switching  is  so  remote  as  not  to  cause  harmful 
transient  disturbances  in  the  communication  circuits. 

k.  Abnormal  Conditions.  A  power  circuit  involved  in  a  parallel 
shall  not  be  operated  at  any  time  with  an  open,  grounded  or  short- 
circuited  line  wire  or  wires  or  transformer  winding. 

1.  Devices  for  Indicating  Abnormal  Conditions  on  Systems  Isolated 
from  Ground.  If  a  power  circuit  involved  in  a  parallel  is  electrically 
isolated  from  ground,  reliable  indicating  devices  "shall  be  installed  at 
its  source  of  supply  to  inform  the  operator  immediately  of  abnormal 
conditions,  such  as  grounds  and  wherever  possible,  open-circuits,  which 
have  not  operated  automatic  switches.  Upon  indication  of  trouble  by 
such  devices,  the  operator  shall  immediately  open  the  oil  switches  and 
proceed  in  the  manner  outlined  in  "m. " 

m.  Procedure  Under  Abnormal  Conditions.  In  case  of  the  opening 
of  an  oil  switch  due  to  an  abnormal  condition  in  a  power  circuit 
involved  in  a  parallel,  or  any  circuit  supplying  or  supplied  by  the 
same,  such  switch  may  be  closed  once;  if  opened  a  second  time  due  to 
the  continuance  of  the  fault  or  abnormal  condition,  said  switch  shall 
not  be  closed  again  until  the  line  has  been  sectionalized.  The  fault  may 
then  be  located  by  energizing  sections  of  line,  provided  that  further 
sectionalization  of  the  line  be  done  in  such  sequence  as  to  cause  the 
minimum  disturbance  to  parallel  communication  circuits,  and  provided 
further  that  where  practicable  the  faulty  section  of  line  shall  be  ener- 
gized but  once  in  this  process  of  sectionalization,  where  the  fault  exists 
within  or  beyond  the  parallel,  until  such  fault  is  remedied. 


14  REPORT    ON    INDUCTIVE    INTERFERENCE. 

n.  Ammeters  in  Neutral  Ground  Connections.  Wherever  a  neutral 
ground  connection  is  employed  on  a  circuit  involved  in  a  parallel  an 
ammeter,  suitable  for  measuring  as  accurately  as  practicable  the  cur- 
rent in  the  neutral  under  normal  operating  conditions,  shall  be  installed 
in  all  neutral  connections  at  the  main  generating  and  substations  on  the 
power  system  electrically  connected  to  the  circuit  involved  in  the 
parallel.  The  power  company  shall  maintain  a  record  of  hourly  meas- 
urements of  the  neutral  current  at  all  such  points. 

o.  Charging  Electrolytic  Lightning  Arresters.  Where  a  power  sys- 
tem is  equipped  with  electrolytic  lightning  arresters  so  charged  as  to 
cause  inductive  interference  in  communication  circuits,  the  method  of 
charging  the  arresters  shall  be  modified  to  eliminate  the  disturbances 
as  far  as  possible.  The  charging  of  such  lightning  arresters  shall  be 
done  at  such  time  as  to  give  the  minimum  liability  of  interference  with 
communication  circuit  operation,  preferably  between  the  hours  of  2  a.  m. 
and  4  a.  m. 

p.  Ware  Form  of  Rotating  Machines.  The  power  company  shall 
make  every  effort  to  obtain  generators  and  synchronous  motors  for  use 
on  all  parts  of  the  system,  giving,  as  nearly  as  rea-:onably  possible,  pure 
sine  waves  of  voltage  at  fundamental  frequency.  In  no  case  shall  the 
deviation  from  a  pure  sine  wave  exceed  the  limit  set  forth  in  the 
Standardization  Rules  of  the  American  Institute  of  Electrical 
Engineers. 

q.  Exciting  Current  of  Transformers.  In  order  that  the  wave 
shapes  of  voltage  and  current  may  be  distorted  as  little  as  practicable 
by  transformers,  the  main  line  transformers  employed  on  circuits  in- 
volved in  a  parallel  and  on  future  extensions  of  such  circuits  shall 
have  an  exciting  current  as  low  as  is  consistent  with  good  practice, 
and  in  no  case  shall  the  exciting  current  at  rated  voltage  exceed  ten 
per  cent  of  the  'full  load  current.  Such  transformers  shall  not  be 
operated  at  more  than  ten  per  cent  above  their  rated  voltage. 

III.  PROVISIONS  APPLYING  TO  EXISTING  PARALLELS. 

The  following  sections  of  II  shall  apply  also  to  power  circuits  involved 
in  existing  parallels:  b,  i,  j,  k,  1,  m,  o,  p,  and  q.  Also,  g-3  and  g-4 
shall  apply  to  existing  parallels  to  the  extent  that  transformers  added 
hereafter  shall  be  connected  as  provided  in  said  rules. 

IV.  WAIVER  OF  CONDITIONS  BY  COMMUNICATION  COMPANY. 

At  the  option  of  the  company  operating  the  communication  circuit  or 
circuits  any  of  the  provisions  of  II  and  III  may  be  waived,  provided 
that  such  waiver  does  not  increase  the  hazard. 

V.  PARALLELISM  WITH  ALTERNATING  CURRENT  RAILWAYS. 

It  is  recognized  that  railroads  operating  alternating  current  trolleys 
with  ground  return  create  serious  inductive  interference  with  parallel 
communication  circuits.  In  the  present  state  of  the  art,  no  means  for 
completely  overcoming  inductive  interference  from  such  parallels  is 
known,  hence,  they  are  to  be  avoided  if  possible  and  where  unavoidable, 
the  responsibilities  arising  therefrom  must  be  settled  by  mutual  agree- 
ment or  in  case  of  inability  to  agree  the  matter  shall  be  referred  to  the 
Railroad  Commission  of  the  State  of  California. 


REPORT    ON    INDUCTIVE   INTERFERENCE.  15 

DISCUSSION   OF   RULES. 

It  will  be  noted  from  the  definitions  that  the  terms  "power  circuit" 
and  ''telephone  circuit"  are  used  in  these  rules  in  a  spiral,  restricted 
sense. 

(I)  The  first  and  most  obvious  means  of  preventing  inductive  inter- 
ference is  to  avoid  the  close  association  of  power  and  communication 
circuits.  Further,  it  is  recognized  that  in  no  other  way  can  complete 
freedom  from  interference  be  secured.  While,  with  the  ever  increasing 
iu1  work  of  electrical  circuits  of  all  kinds,  adequate  separation  to  avoid 
interference  is  becoming  increasingly  difficult  to  maintain,  the  com- 
mittee feels  that  the  importance  of  such  separation  justifies  its 
being  made  the  first  premise  in  rules  designed  to  prevent  inductive 
interference. 

Notice,  sufficiently  in  advance,  should  be  given  the  other  party  or 
parties  concerned  in  any  proposed  parallel  in  order  that  thorough 
consideration  may  be  given  by  both  parties  to  possible  means  of 
avoiding  the  parallel  or,  in  case  the  parallel  can  not  be  avoided,  to  the 
necessary  remedial  measures  to  be  employed. 

(Il-a)  The  best  insurance  against  physical  hazard  in  case  of  close 
proximity  is  to  maintain  a  separation  equal  to  the  height  of  the  taller 
line,  thus  avoiding  the  possibility  of  physical  contact  in  case  of  failure. 
In  the  case  of  crossings  and  unavoidable  cases  of  close  proximity  for 
short  distances  extra  strength  construction  is  necessary  as  a  precaution 
against  failure. 

(II-b-c)  As  has  been  pointed  out  under  the  heading  "Results  Ac- 
complished," and  more  fully  explained  in  Appendix  II,  residual 
voltages  and  currents  are  particularly  troublesome  factors  in  causing 
interference.  Means  to  eliminate  or  reduce  such  residuals  in  power 
systems  are  highly  important  and  while  information  at  this  time  does 
not  enable  the  committee  to  formulate  as  explicit  a  rule  as  is  desirable, 
yet  the  importance  of  the  subject  justifies  its  inclusion  in  the  rules. 
The  acquisition  of  further  information  on  which  to  base  a  more  explicit 
rule  upon  this  subject  is  a  most  important  problem,  the  experimental 
study~of  which  is  discussed  in  the  following  section  of  this  report. 

(Il-d)  Transpositions  properly  located  in  both  power  and  communica- 
tion circuits  offer  the  most  reliable  and  effective  means  for  preventing 
interference  from  balanced  voltages  and  currents  of  power  circuits. 
While  the  inductive  effects  increase  in  severity  for  the  higher  voltage 
circuits,  due  in  part  to  the  increased  separation  of  the  line  conductors, 
which  renders  more  frequent  transpositions  desirable,  the  mechanical 
difficulties  involved  are  so  great  as  to  overbalance  the  other  reasons  and 
the  rules,  therefore,  provide  for  less  frequent  transpositions  in  the 
higher  voltage  circuits  than  in  the  lower  voltage 'circuits.  A  further 
reason  for  frequent  transpositions  in  the  lower  voltage  circuits  is  the 
necessity  of  a  flexible  system  of  transpositions  applicable  to  short 
parallels  which  generally  occur  with  such  circuits. 

(Il-e)  The  provision  requiring  transpositions  outside  the  limits  of  a 
parallel  on  systems  electrically  isolated  from  ground  is  an  explicit 
measure  for  carrying  out  the  purpose  of  the  more  general  provision 
given  under  II-b-c,  "Balance  of  Power  System"  and  "Limitation  of 
Residual  Voltages  and  Currents." 

3—12191 


16  REPORT  ON  INDUCTIVE  INTERFERENCE. 

(Il-f )  Non-uniformity  of  separation  and  type  of  construction  within 
the  limits  of  a  parallel  are  inequalities  which  can  not  in  many  cases  be 
taken  into  account  in  the  design  and  layout  of  transposition  schemes. 
Such  inequalities  tend  to  nullify  the  effectiveness  of  the  transpositions, 
hence  it  is  desirable  that  they  be  avoided.  A  precautionary  statement 
is  included  in  the  rule  in  order  that  the  possibility  of  securing  a  wide 
separation  for  a  considerable  portion  of  a  parallel  may  not  be  sacrificed 
for  the  sake  of  absolute  uniformity  throughout  the  entire  length. 

(Il-g)  Some  types  of  transformer  connections  and  methods  of  opera- 
tion give  rise  to  large  residual  voltages  and  currents  and  certain  pro- 
visions of  the  rules  are  designed  to  prohibit  or  restrict  the  use  of  such 
connections  and  methods  of  operation.  The«e  rules  may  be  considered 
as  explicit  provisions  complying  with  the  general  provision  in  II-b-c, 
"Balance  of  Power  System"  and  "Limitation  of  Residual  Voltage  and 
Currents."  The  sufficiency  of  these  specific  provisions  as  an  insur- 
ance against  harmful  residual  voltages  and  currents  is  subject  to 
future  determination. 

The  present  information  of  the  committee  does  not  warrant  the 
definite  recommendation  of  any  one  type  of  connection  or  method  of 
operation  as  best  from  the  standpoint  of  inductive  interference.  This 
is  true  as  to  the  relative  merits  of  the  two  general  types  of  systems, 
the  grounded  neutral  and  the  isolated  system.  The  advantages  and 
di-advantages  of  these  general  types  and  any  modifications  of  these 
types  are  dependent  upon  their  inherent  characteristics  in  respect  to 
residuals  and  the  limitations  and  control  of  residuals  under  both  normal 
and  abnormal  conditions.  Both  types  are  on  an  equality  with  respect 
to  the  interference  caused  by  balanced  voltages  and  currents. 

(Il-k)  Continued  operation  under  certain  abnormal  conditions  is 
possible  in  some  power  systems.  In  particular,  it  is  possible  to  operate 
a  grounded  star-connected  system  with  one  phase  open,  and  it  is 
possible  to  continue  the  operation  of  an  isolated  system  when  one  phase 
becomes  grounded  accidentally.  The  former  gives  rise  to  a  large  resid- 
ual current  and  the  latter  to  a  large  residual  voltage,  both  of  which  are 
liable  to  render  parallel  communication  circuits  inoperative.  For  these 
reasons  the  rule  prohibits  such  operation  which,  aside  from  the  con- 
sideration of  inductive  interference,  does  not  constitute  good  practice  in 
power  system  operation. 

(II-h-1-n)  To  provide  that  operation  under  the  abnormal  conditions 
mentioned  above  may  not  continue  without  the  knowledge  of  the  power 
company,  the  rules  specify  that  devices  for  indicating  grounds  shall  be 
installed  on  isolated  systems.  With  respect  to  grounded  star-connected 
systems,  the  rules  specify  with  certain  exceptions  the  automatic  opening 
of  switches  by  abnormal  neutral  currents.  In  such  systems  ammeters 
are  required  in  all  main  neutral  ground  connections.  Such  ammeters, 
read  regularly,  afford  means  of  detecting  abnormal  neutral  currents 
and  are  of  value  in  showing  the  degree  of  balance  of  the  system,  as  the 
neutral  current  is  easily  affected  by  unbalanced  conditions. 

(Il-m)  Accidental  causes  give  rise  to  occasional  abnormal  conditions. 
These  can  only  be  guarded  against  by  good  construction  and  mainten- 
ance, and  careful  operation  which,  however,  can  not  prevent  entirely 
such  occurrences.  When  trouble  develops  on  a  power  circuit  involved 
in  a  parallel,  it  is  always  liable  to  cause  serious  interference  to  the 


REPORT    ON    INDUCTIVE   INTERFERENCE.  17 

coirimunication  circuits,  if  the  exposure  is  severe.  In  the  present  state 
of  the  art,  the  method  of  fault  location  on  power  circuits  is  a  process  of 
repeated  sectionalization  and  energization  of  the  faulty  line  until  the 
fault  is  located  within  certain  limits.  This  process  causes  repeated 
interruptions  with  loss  of  time  in  the  operation  of  the  communication 
circuits,  and  in  the  case  of  telephone  circuits  is  accompanied  sometimes 
by  injury  to  the  operators.  It  should  be  explained  that  the  loss  of  time 
is  much  greater  than  the  duration  of  the  disturbance,  owing  to  the  time 
required  to  restore  the  protective  devices  on  the  communication  circuits 
to  their  normal  condition.  No  method  of  locating  faults  on  power  cir- 
cuits is  known  which  meets  the  requirements  of  practice  and  yet  avoids 
the  disadvantages  of  the  present  method.  The  inductive  disturbances 
due  to  fault  location  can  be  to  a  considerable  degree  ameliorated  by 
disconnecting  the  faulty  line  from  the  rest  of  the  system  and  energizing 
this  line  by  a  single  generator  at  such  excitation  as  may  be  necessary 
to  overcome  the  insulation  of  the  fault.  Whenever  practicable  this 
method  is  employed  by  power  companies ;  hence,  it  has  not  been  thought 
necessary  to  cover  it  by  a  specific  rule. 

In  view  of  these  facts,  the  committee  is  recommending  the  limitation 
of  the  present  practice  in  this  regard  so  as  to  avoid,  as  far  as  seems 
practicable,  the  repeated  interruptions  to  communication  circuit  opera- 
tion. It  is  highly  desirable  that  some  better  method  of  fault  location 
be  developed,  not  only  because  of  the  attendant  consequences  of  the 
present  method  on  communication  circuits,  but  also  because  of  the 
abnormal  strains  to  which  the  power  apparatus  is  necessarily  subjected. 

(II-h-i-j)  Normal  switching  operations  on  power  circuits  produce  at 
times  severe  transient  disturbances  in  parallel  communication  circuits. 
The  commonly  recognized  fact  that  oil  switches  produce  less  severe 
transient  disturbances  in  power  circuits,  affords  the  basis  for  the  pro- 
visions in  the  rules  dealing  with  switches  and  switching.  The  auto- 
matic features  required  are  designed  to  prevent  continued  operation 
under  abnormal  conditions. 

(II-o)  Transient  disturbances  of  severe  nature  to  telephone  circuits 
are  sometimes  caused  by  the  charging  of  electrolytic  lightning  arresters. 
There  are  available  methods  of  diminishing  the  transients  due  to  this 
cause,  and  a  general  provision  to  the  effect  that  such  methods  shall  be 
employed  when  necessary  is  included  in  this  rule.  It  is  further  pro- 
vided that  the  charging  of  arresters  should  be  done  at  times  when  the 
telephone  circuits  are  least  used. 

(II-p-q)  Fundamentally,  interference  to  telephone  circuits  by  power 
circuits  in  normal  operation  is  largely  due  to  the  existence  of  harmonics 
in  the  currents  and  voltages.  While  the  complete  elimination  of  these 
harmonics  seems  impracticable,  still  beneficial  results  may  be  obtained 
by  practical  efforts  in  this  direction,  and  the  committee  feels  that  the 
two  general  provisions  as  to  the  wave  form  of  rotating  machines  and 
the  exciting  current  of  transformers  are  of  great  importance  both  from 
a  practical  standpoint  and  also  as  enunciating  a  general  principle. 
The  matter  of  generator  wave  form  particularly  is  of  importance  for 
all  types  of  systems.  The  provision  with  reference  to  the  exciting 
current  of  transformers,  while  desirable  in  all  cases,  is  particularly 
so  on  grounded  star-connected  systems. 


18  REPORT    ON   INDUCTIVE   INTERFERENCE. 

(III)  Certain  of  the  measures  in  II,  particularly  those  referring  to 
power  system  operation,  which  are  helpful  in  mitigating  inductive  inter- 
ference have  been  recommended  to  apply  to  existing  parallels. 

(IV)  Since  these  rules  are  designed  for  the  protection  of  communica- 
tion circuits,  it  is  proper  that  the  companies  operating  such  circuits  be 
given  the  right  to  waive  any  measures  of  protection  which  they  may 
in  any  particular  case  consider  unnecessary. 

(V)  The  committee  has  undertaken  no  investigation  of  cases  of  par- 
allelism with  alternating  current  railways,  but  as  the  seriousness  of  this 
class  of  exposure  is  recognized,   it  was  thought  desirable  that  it  be 
referred  to  specifically. 

FUTURE  WORK. 

The  further  work  necessary  in  order  to  secure  the  information  essen- 
tial as  a  basis  of  determining  more  explicit  and  effective  rules  than 
those  herein  recommended,  is  particularly  concerned  with  the  subjects 
of  transpositions  and  residual  voltages  and  currents.  In  order  to  cover 
these  subjects  in  as  effective  and  economical  a  manner  as  possible  it  is 
thought  that  the  procedure  should  be  along  the  following  lines: 

1.  Experimental  study  of  transpositions  which  includes  the  deter- 
mination of : 

(a)  The  practical  effectiveness  of  transpositions  in  both  power 
and  communication  circuits  as  a  means  of  reducing  induction  aris- 
ing from  balanced  voltages  and  currents ;  involving  considerations 
of  different  co-ordinated  transposition  schemes,  particularly  with 
different  lengths  of  power  circuit  barrels. 

(6)  The  practical  effectiveness  of  transpositions  in  communica- 
tion circuits  as  a  means  of  reducing  inductive  interference  arising 
from  residual  voltages  and  currents ;  involving  considerations  of 
different  systems,  particularly  different  lengths  of  balanced  com- 
munication circuit  transposition  sections. 

(<?)  The  influence  of  imperfect  electrical  balance  of  communica- 
tion circuits  in  impairing  the  effectiveness  of  transpositions. 

(d)  The  practical  effectiveness  of  transpositions  in  a  power 
circuit  isolated  from  ground  as"  a  means  of  balancing  the  electro- 
static capacities  to  earth  of  the  several  conductors,  and  thereby 
reducing  residual  voltages  and  currents;  involving  considerations 
of  the  relative  efficiency  of  different  lengths  of  power  circuit  barrels. 

2.  Experimental    study    of    the    causes    and    effects    of    residuals, 
including : 

(a)  A  comparison  of  the  different  types  of  power  system  con- 
nection and  apparatus  in  common  use  and  their  characteristics  in 
respect  to  the  production  of  residuals,  particularly  harmonic 
residuals. 

(5)  Means  to  be  employed  in  limiting  residual  voltages  and 
currents. 

( c )  A  determination  of  the  minimum  values  of  residual  voltages 
and  currents  which  will  produce  harmful  inductive  interference. 

It  is  thought  that  these  two  studies  could  progress  simultaneously. 
The  work  indicated  under  (1)  could  best  be  done  on  an  actual  parallel 
selected  to  be  as  uniform  and  as  free  from  secondary  disturbances  as 
possible.  Some  preliminary  work  has  been  done  along  these  lines 


REPORT    ON    INDUCTIVE    INTERFERENCE.  19 

which  indicates  the  best  methods  of  procedure,  and  this  should  facili- 
tate the  carrying  out  of  the  investigation. 

The  study  mentioned  under  (2)  consists  in  part  of  an  investigation 
of  the  characteristics  and  magnitudes  of  residual  voltages  and  currents 
in  typical  power  systems,  both  those  with  grounded  neutrals  and  sys- 
tems entirely  isolated  from  ground.  A  part  of  the  study  of  residuals 
is  logically  related  to  the  study  of  transpositions  and  could  be  carried 
out  in  connection  with  the  study  outlined  under  (1)  and  at  the  same 
time  and  place. 

In  addition  to  the  above  the  committee  has  already  arranged  for  the 
investigation  of  the  two  following  subjects: 

1.  A  determination  of  the  detrimental  effect  of  extraneous  cur- 
rents on  a  telephone  circuit  as  a  function  of  the  frequency  includ- 
ing a  determination  of  the  maximum  amount  of  extraneous  current, 
of  different  frequencies  and  combinations  of  frequencies,  which  is 
allowable  in  a  commercial  telephone  circuit. 

2.  A  determination  of  the  effects  of  extraneous  current  of  differ- 
ent amounts  and  characteristics,  in  limiting  the  speed  of  telegraph 
operation. 

This  work  is  now  in  progress. 


20  REPORT    ON    INDUCTIVE   INTERFERENCE. 


APPENDIX  I. 

HARMONICS. 

Any  complex  electrical  wave  of  periodic  .structure  may  be  resolved 
into  component  sine  waves  of  suitable  amplitudes  and  phase  differences, 
having  frequencies  which  are  in  integral  relation  to  the  fundamental 
frequency.  The  simple  sine  wave  of  lowest  frequency,  is  termed  the 
fundamental,  and  those  of  higher  frequency  are  termed  harmonics 
of  the  fundamental  wave.  The  fundamental  may  be  considered  the 
first  harmonic.  The  analysis  of  a  periodic  wave  into  its  constituent 
sine  waves  or  harmonics  is  not  merely  a  mathematical  conception  or 
process  but  is  in  accordance  with  the  facts  of  electricity  and  acoustics. 

In  general,  alternating  current  systems,  by  virtue  of  their  inherent 
characteristics,  do  not  permit  the  existence  of  harmonics  other  than 
odd  integral  mulitples  of  the  fundamental  frequency,  i.  e.,  3d,  5th,  7th, 
9th,  llth,  etc.,  harmonics.  Such  harmonics  may  exist  in  either  or  both 
the  current  and  voltage  waves  of  a  power  system. 

Commercial  frequencias  of  power  transmission  in  California  are  25, 
50  and  60  cycles  per  second.  The  power  systems,  so  far  investigated, 
operate  at  a  fundamental  frequency  of  60  cycles  per  second.  The 
investigation  has  shown  harmonic  currents  and  voltages  of  appreciable 
magnitude  up  to  the  35th  harmonic.  On  one  system  the  23d  (corre- 
sponding to  a  frequency  of  1,380  cycles  per  second)  has  been  found  to 
be  prominent.  Induced  currents  and  voltages  in  parallel  communi- 
cation circuits  have  been  observed  corresponding  to  these  harmonics. 

The  detrimental  effect  of  the  induced  voltages  and  currents  in  parallel 
communication  circuits  depends,  in  general,  upon  their  magnitude  and 
upon  the  frequency  of  the  induction  as  compared  with  the  operating 
frequency  of  the  communication  circuit.  The  presence  of  extraneous 
current  of  a  frequency  approaching  that  of  normal  operating  frequency 
of  the  communication  circuit  has  a  more  injurious  effect  than  the  same 
amount  of  current  of  a  frequency  far  removed  from  the  operating 
frequency  of  the  circuit. 

The  frequency  of  the  voice  currents  flowing  in  a  telephone  circuit 
ranges  from  about  200  cycles  per  second  up  to  possibly  2000  cycles  per 
second.  The  average  voice  frequency  is  considered  to  be  approximately 
800  cycles  per  second,  and  at  about  this  frequency  the  telephone  receiver 
is  most  sensitive.  It  is  on  account  of  these  considerations  that  extra- 
neous currents  of  the  higher  frequencies,  arising  from  the  harmonics 
of  a  power  system,  are  relatively  more  detrimental  to  telephone  service. 
The  harmonics  of  the  power  systems  have  been  found  to  be  responsible 
for  the  greater  portion  of  the  inductive  .interference  to  telephone 
service,  under  normal  operating  conditions  of  parallel  power  circuits. 
Any  extraneous  current  of  a  frequency  within  the  audible  range  pro- 
duces a  disturbance  which  impairs  the  efficiency  of  a  telephone  circuit. 
The  combined  effects  of  all  extraneous  currents  present,  of  frequencies 
within  the  range  of  audition,  constitute  the  humming  "noise"  heard 
in  the  receiver  of  a  telephone  circuit  which  is  subject  to  induction. 

The  effect  of  currents  of  the  fundamental  frequency  (60  cycles  or 
less)  on  telephone  circuits  is  relatively  unimportant  as  compared  to  that 
of  higher  harmonics,  owing  to  the  fact  that  the  fundamental  approaches 
the  lower  limit  of  audible  frequencies.  However,  if  the  induction  due 


REPORT    ON    INDUCTIVE    INTERFERENCE.  21 

to  the  fundamental  becomes  sufficiently  great,  constituting  a  physical 
hazard,  or  of  such  magnitude  as  to  operate  the  protective  devices  on 
the  telephone  circuits  or  interfere  with  superimposed  telegraph  service 
or  other  grounded  signalling  devices,  it  is  then  of  great  importance 
from  the  standpoint  of  interference. 

In  regard  to  the  effect  of  extraneous  currents  on  the  operation  of 
telegraph  circuits,  for  reasons  analogous  to  those  given  above,  such 
circuits  are  relatively  more  affected  by  extraneous  currents  of  funda- 
mental frequency  or  of  the  frequencies  corresponding  to  the  lower  har- 
monics such  as  the  3d  and  5th. 

At  the  present  time  the  American  Telephone  and  Telegraph  Company 
is  undertaking,  on  behalf  of  the  Joint  Committee  on  Inductive  Interfer- 
ence,  an  extensive  series  of  tests  in  regard  to  the  detrimental  effect  of 
extraneous  currents  of  various  frequencies  on  the  intelligibility  of  tele- 
phone conversation.  In  addition,  this  company,  in  conjunction  with 
the  Western  Union  Telegraph  Company  and  the  Postal  Telegraph 
Cable  Company,  is  undertaking  an  investigation  of  the  effect  of  extran- 
eous currents  on  the  operation  of  telegraph  circuits  and  apparatus  of 
different  types. 

Harmonic  currents  and  voltages  in  power  circuits  arise  from  many 
causes.  Generators  or  other  rotating  machines  do  not,  in  general,  pro- 
duce pure  sine  waves  of  fundamental  frequency.  This  is  due  to  several 
features  in  the  design  of  the  apparatus.  A  certain  amount  of  distortion 
of  wave  form,  with  the  consequent  introduction  of  disturbing  harmonics, 
is  inherent  with  the  use  of  transformers.  This  distortion  of  wave  form 
is  due  to  hysteretic  action  in  the  iron  core  of  the  transformer.  The 
distortion  varies  in  character  and  magnitude  with  the  saturation  and 
characteristics  of  the  iron  employed.  Certain  connections  of  trans- 
formers are  possible  which  will  suppress  the  third  harmonic  and  its 
multiples  in  a  three-phase  power  system.  The  fact  that  practically  all 
inductive  interference  to  telephone  circuits  is  due  to  the  harmonic 
currents  and  voltages,  renders  it  important  that  an  effort  be  made  to 
obtain  rotating  machinery  for  use  in  power  systems  which  produces  as 
nearly  as  is  reasonably  possible  pure  sine  waves  of  fundamental  fre- 
quency, and  also  that  an  effort  be  made  to  obtain  transformers  and  to 
arrange  connections  of  the  same  in  such  a  manner  as  to  reduce  as  far 
as  practicable  the  distortion  of  wave  form. 


22  REPORT    ON    INDUCTIVE    INTERFERENCE. 

APPENDIX  II. 

BALANCED    AND    RESIDUAL    VOLTAGES    AND    CURRENTS. 

This  appendix  comprises  the  four  following  sections: 

1.  Analysis  of  Voltages  and  Currents  and  Discussion  of  the  Effects  of  their 

Components. 

2.  Causes  of  Residual  Voltages  and  Currents. 

3.  Means  for  Preventing  or  Reducing  Residual  Voltages  and  Currents. 

4.  Discussion  of  Tests. 

1.     Analysis    of    Voltages    and     Currents    and     Discussion    of    the     Effects    of    Their 
Components. 

To  facilitate  the  analysis  of  inductive  effects  in  parallel  communica- 
tion circuits,  arising;  from  a  power  circuit,  the  voltages  and  currents  of 
the  power  circuit  can  be  conveniently  regarded  as  consisting  of  com- 
ponents which  exhibit  distinct  characteristics  and  which  may  be  treated 
separately. 

Considering  a  three-phase  circuit  having  equal  voltages  between  any 
two  conductors,  the  vcltages  to  ground  from  the  conductors  can  be 
resolved  into  two  sets  of  components,  balanced  components  and  residual 
components.  Since  the  voltages  between  any  two  conductors  are  equal, 
the  voltages  between  the  conductors  may  be  graphically  represented  by 
three  vectors  forming  an  equilateral  triangle.  The  potential  of  the 
ground  may  be  represented  by  a  point  which  may  be  inside  or  outside 
of  the  triangle  depending  on  the  magnitude  and  character  of  the  resi- 
dual voltage,  and  the  actual  voltages  to  ground  from  the  conductors 
may  be  represented  by  three  vectors  drawn  between  the  point  represent- 
ing the  ground  potential  and  the  corners  of  the  triangle.  The  balanced 
components  of  the  voltages  to  ground  from  the  conductors  consist  of 
three  equal  voltages  whose  vector  sum  is  zero  and  which  are  therefore 
displaced  one  third  cycle  in  time  phase  with  respect  to  one  another. 
These  balanced  components  may  be  represented  by  three  vectors  drawn 
from  the  center  of  the  equilateral  triangle  to  the  corners.  The  residual 
components  of  the  voltages  to  ground  from  the  conductors  consist  of 
three  equal  voltages  wrhich  are  in  phase  with  one  another  and  which  may 
be  represented  by  three  identical  vectors  drawn  from  the  point  repre- 
senting the  ground  potential  to  the  center  of  the  equilateral  triangle. 
If  the  residual  voltage  is  zero  the  point  representing  the  ground  poten- 
tial will  be  at  the  center  of  the  triangle.  The  residual  voltage  of  the 
system  is  denned  as  the  vector  sum  of  the  voltages  of  the  three  conduc- 
tors to  ground.  It  is,  therefore,  by  definition,  three  times  the  residual 
voltage  of  the  individual  conductors,  or  three  times  the  equivalent 
single-pha^e  voltage  of  the  three  conductors  in  parallel  with  respect  to 
the  earth.  It  should  be  noted  that  the  inductive  effect  of  the  residual 
voltage  is  equal  to  that  of  a  single-phase  voltage  between  ground  and 
the  three  conductors  in  parallel,  equal  to  the  residual  voltage  of  the 
individual  conductors,  or  to  one  third  the  residual  voltage  of  the 
system. 

If  one  conductor  is  grounded  the  residual  components  (assuming  the 
voltages  between  wires  remain  unchanged)  will  each  equal  the  voltage 
between  conductors  divided  by  the  square  root  of  three,  and  the  residual 
voltage  of  the  system  will  be  equal  to  the  voltage  between  conductors 
multiplied  by  the  square  root  of  three. 


REPORT    ON    INDUCTIVE    INTERFERENCE.  23 

If  a  power  circuit  consists  of  a  single  conductor  with  ground  return, 
the  residual  voltage  will  be  equal  to  the  voltage  from  the  conductor  to 
ground. 

The  currents  flowing  in  the  three  wires  of  a  three-phase,  three-wire 
circuit  can  be  considered  to  be  composed  of  three  sets  of  currents ; 
namely,  (1)  balanced  components  consisting  of  equal  currents  in  each 
of  the  three  line  wires  whose  vector  sum  is  zero,  and  which  are,  there- 
IV iv.  displaced  one  third  cycle  in  time-phase  with  respect  to  one  another; 
(2)  a  single-phase  current  flowing  in  a  loop  compcsed  of  two  of  the 
line  wires;  (3)  a  residual  current  divided  equally  between  the  three 
line  wires  and  returning  through  the  earth.  The  residual  current  of 
the  three-phase  circuit  is  defined  as  the  vector  sum  of  the  three  line 
currents.  It  is,  therefore,  the  equivalent  of  a  single-phase  current 
flowing  through  the  three  line  conductors  in  parallel,  with  the,  earth 
completing  the  circuit. 

In  the  ca?e  of  a  power-  circuit  consisting  of  a  single  conductor  with  a 
ground  return  the  entire  current  flowing  in  the  conductor  is  residual. 

In  the  above  discussion,  reference  is  made  to  three-phase,  three-wire 
power  circuits,  but  the  analysis  there  given  may  be  generalized  so  as 
to  apply  to  a  power  system  of  any  number  of  phases.  Most  electrical 
power  transmission  systems  are  of  the  three-phase,  three-wire  type  and 
subsequent  statements  will  apply  particularly  to  such  systems,  unless 
otherwise  stated. 

At  a  point  in  the  vicinity  of  a  power  circuit,  such  as  might  represent 
the  location  cf  an  element  of  a  communication  circuit  conductor,  the 
resultant  electromagnetic  field  due  to  the  balanced  currents  would  be 
zero  if  the  power  circuit  conductors  were  equidistant  from  the  point 
(disregarding  the  effect  of  the  earth).  In  general,  the  power  circuit 
conductors  are  not  exactly  equidistant  from  such  point,  and  therefore 
the  resultant  electromagnetic  field  due  to  balanced  currents  is  not  zero. 
For  this  reason,  the  balanced  currents  in  the  power  circuit  have  un- 
equal effects  on  the  communication  circuit,  hence-  there  is  a  resultant 
induction.  For  residuals,  there  is,  in  general,  a  much  greater  inequality 
in  the  distances  between  the  affected  conductors  (or  circuits)  and  the 
sides  of  the  residual  circuit  (power  conductors  in  parallel  one  side,  earth 
other  side)  than  in  the  distances  to  the  several  power  conductors,  which 
constitute  the  circuit  for  the  balanced  components.  Thus  the  resultant 
electromagnetic  field  due  to  residual  currents  is  large  in  comparison 
with  the  field  set  up  by  balanced  currents  of  the  same  magnitude.  It 
may  be  noted  that  the  electromagnetic  forces  at  any  point  due  to 
residual  currents  in  the  different  power  conductors  are  in  the  same  time- 
phase,  hence  the  inductive  effects  of  all  the  residual  components  are 
cumulative  and  not  differential  as  in  the  case  of  the  balanced 
components. 

In  a  similar  way  it  may  be  shown  that  residual  voltages  produce 
proportionately  far  greater  inductive  effects  than  balanced  voltages. 

Computations  based  on  the  physical  characteristics  of  two  of  the 
parallels  investigated  show  that,  for  an  exposure  near  Salinas  for  eight 
miles  with  a  55,000-volt  line  on  the  opposite  side  of  the  county  road 
from  a  communication  line,  one  ampere  of  residual  current  produces  as 
much  induction  in  a  ground  return  communication  circuit  as  would 
forty  amperes  of  balanced  current;  and  one  volt  residual  produces  as 
much  induction  as  one  hundred  and  ten  volts  balanced.  Similar  com- 


24  REPORT    ON   INDUCTIVE   INTERFERENCE. 

putations  based  on  the  physical  characteristics  of  an  exposure  between 
Santa  Cruz  and  Watsonville,  where  the  communication  circuits  are 
paralleled  for  seventeen  miles  by  a  22,000-v.olt  line  on  the  opposite  side 
of  the  county  road,  show  that  one  ampere  residual  produces  as  much 
induction  in  a  ground  return  communication  circuit  as  would  two 
hundred  and  forty  amperes  of  balanced  current ;  and  one  volt  residual 
produces  as  much  induction  as  ten  volts  balanced.  All  of  the  above 
comparative  values  are  for  currents  and  voltages  of  sixty  cycles 
frequency. 

The  above,  values  illustrate  the  relative  induction-producing  powers  of 
balanced  and  residual  currents  and  voltages  in  two  specific  cases.  Such 
values  will  vary  considerably  for  different  p-arallels,  but  these  cited  may 
be  taken,  in  a  general  way,  as  indicative  of  the  relative  severity  of  the 
effects  on  a  single  conductor  produced  by  these  two  factors.  Such 
values  for  a  unit  length  of  non-transposed  circuit  in  any  given  parallel, 
are  dependent  upon  the  separation,  height,  and  configuration  of  the 
conductors  of  the  two  classes  of  circuits,  and  upon  the  character  and 
condition  of  the  ground  and  neighboring  objects.  For  the  entire 
parallel,  or  total  length  of  exposure,  these  values  are  further  dependent 
upon  transpositions.  The  actual  amount  of  induction  arising  from 
each  of  the  two  components  depends  also  upon  the  actual  magnitudes 
and  the  frequencies  of  the  components  in  the  power  circuit. 

It  will  be  shown  in  Appendix  III  that  inductive  interference  arising 
from  balanced  currents  and  voltages  can  be  reduced  by  proper  transpo- 
sitions in  the  power  circuit,  but  that  power  circuit  transpositions  do 
not  reduce  the  inductive  interference  produced  in  a  parallel  communi- 
cation circuit  by  residuals.  Residual  currents  and  voltages  act  induc- 
tively to  produce  the  same  effects  as  a  single-phase  grounded  circuit 
operating  with  the  three  line  conductors  in  parallel.  This  generally 
represents  the  worst  possible  condition  from  the  standpoint  of  inductive 
interference.  Transposing  the  conductors  of  the  power  circuit  can  not 
reduce  the  inductive  interference  arising  from  residuals,  except  in  so 
far  as  the  magnitude  of  the  residual  voltages  and  currents  is  reduced 
by  such  transpositions.  The  effect  of  power  circuit  transpositions  on 
the  magnitude  of  these  components  is  discussed  below. 

In  -the  detailed  discussion  of  transpositions  in  Appendix  III  it  is 
shown  that  transpositions  in  a  communication  circuit  can  reduce  the 
induced  voltages  from  residuals  only  as  between  the  two  sides  of  a 
metallic  circuit. 

In  view  of  the  above  it  is  evident  that  attention  must  be  given  to  the 
problem  of  restricting  residuals  to  amounts  which  do  not  cause  material 
interference  either  to  grounded  communication  circuits  or  to  properly 
transposed  and  balanced  metallic  circuits. 

2.     Causes  of  Residual  Voltages  and  Currents. 

While  a  degree  of  balance  of  the  voltages  and  currents  of  the  power 
system  may  be  obtained  which  satisfies  all  the  practical  demands  of 
power  operation,  this  may  not  be  sufficient  to  prevent  the  production  of 
residuals  sufficient  to  cause  serious  inductive  interference  to  parallel 
communication  circuits. 


REPORT  ON  INDUCTIVE  INTERFERENCE.  25 

Residual  currents  and  voltages  may  arise  from  one  or  more  causes 
which  act  singly  or  together.  The  principal  sources  of  residual  cur- 
rents and  voltages  are, 

1.  Unbalanced  loads  between  the  three  phases  and  the  neutral 
of  a  grounded  star-connected  system. 

2.  The  introduction  of  the  third  harmonic  and  its  odd  multiples 
as  residual  current  and  voltage  due  to  certain  apparatus  and  con- 
nections employed  on  a  grounded  star-connected  system. 

3.  Unbalanced  capacity  and  leakage  between  the  several  phases 
of   the   system   and   ground.     This   applies   more   particularly   to 
systems  isolated  from  ground. 

There  are  two  principal  types  of  commercial  three-phase  power  cir- 
cuits used  in  California. 

1.  The  grounded  neutral  circuit  or  network,  in  which  all  import- 
ant generating  points  have  a  grounded  neutral  and  in  which  all  or 
part  of  the  receiving  points  may  be  connected  with  a  grounded 
neutral.     No   resistances   are   inserted  between  the  neutrals   and 
ground. 

2.  The  isolated  circuit  or  network,  which  normally  has  no  metal- 
lic connection  to  ground  at  any  point. 

The  characteristics  of  the  grounded  neutral  system  with  particular 
reference  to  residuals  are  as  follows : 

UNDER  NORMAL  CONDITIONS. 

(a)  The  impedances  between  line  conductors  and  ground  are  de- 
termined very  largely  by  the  load  impedances  of  the  transformers. 
With  balanced  loads  the  residual  voltage  other  than  the  third  harmonic 
and  its  odd  multiples  may  be  eliminated. 

(6)  The  effect  of  unbalanced  loads  on  the  residual  voltage  is  small, 
as  the  tendency  of  generators  and  transformers  is  to  maintain  equal 
voltages  between  the  several  conductors  and  ground. 

(c)  With  balanced  loads  the  residual  current,  other  than  the  third 
harmonic  and  its  odd  multiples,  may  be  eliminated. 

(d)  Unbalanced  loads  between  line  and  neutral  cause  corresponding 
residual  currents,  which  will  be  large  if  the  unbalance  is  large,  as  such 
unbalanced  load  currents  flow  through  the  neutral  to  earth. 

(e)  The  varying  permeability  of  the  iron  in  star-connected  trans- 
formers with  grounded  neutrals  introduces  the  third  harmonic  and  its 
odd  multiples  as  residual  voltages  and  currents.     The  use  of  delta- 
connected  secondary  windings  reduces  this  effect  greatly  below  that  of 
star  to  star-connections. 

(/)  Grounded  star-connected  generators  connected  directly  to  the 
line  or  through  grounded  star  to  star-connected  banks  of  transformers, 
may  introduce  the  third  harmonic  and  its  odd  multiples  as  residual 
voltages  and  currents. 

UNDER  ABNORMAL  CONDITIONS. 

(g)  A  ground  on  one  phase  short-circuits  that  phase  through  the 
neutral  connection  and  causes  a  residual  current  throughout  the  whole 
length  of  the  circuit,  this  current  being  practically  equal  to  the  short- 
circuit  current  to  ground  on  that  portion  of  the  circuit  between  the 
sources  of  power  supplying  the  fault  and  the  point  where  the  circuit  is 
grounded.  A  large  residual  voltage  (approaching  as  maximum  58  per 


26  REPORT    ON    INDUCTIVE    INTERFERENCE. 

cent  of  the  voltage  between  phases)  will  be  created  in  proximity  to  the 
fault  and,  if  the  low  tension  side  of  the  receiving  transformers  is  star- 
connected,  throughout  that  portion  of  the  circuit  between  the  fault  and 
such  receiving  transformers.  If  the  neutral  of  the  receiving  trans- 
formers is  isolated,  the  short-circuit  current  will  exist  only  between  the 
source  of  supply  and  the  fault  and  there  will  be  no  residual  current 
between  the  fault  and  such  receiving  transformers.  The  above  men- 
tioned residual  voltage  will  in  this  case  exist  not  only  in  proximity  to 
the  fault  on  the  supply  side  but  also  throughout  the  length  of  circuit 
from  the  fault  to  the  receiving  transformers.  The  power  circuit  is 
rendered  inoperative. 

(h)  An  open  condition  of  one  phase  causes  a  large  residual  current, 
as  the  unbalanced  load  currents  of  the  other  two  phases  must  flow 
through  the  neutral  to  earth.  A  large  residual  voltage  will  exist  beyond 
the  fault  if  the  low  tension  side  of  the  receiving  transformers  is  star- 
connected.  The  power  circuit  may  not  be  rendered  inoperative  for 
three-phase  supply  be3rond  the  fault,  in  case  the  receiving  transformers 
are  grounded  star-delta  connected. 

The  characteristics  of  the  isolated  system  with  particular  reference 
to  residuals  are  as  follows : 

UNDER  NORMAL  CONDITIONS. 

(a)  The  impedances  between  line  conductors  and  ground  are  de- 
termined by  the  electrostatic  capacities  and  the  leakage  between  the 
several  conductors  and  ground.  With  balanced  loads  a  residual  voltage 
may  exist,  due  to  unbalanced  capacity  and  leakage.  Such  residual  vol- 
tage as  is  due  to  unbalanced  capacity  may  be  eliminated  by  transposing 
the  circuit  so  as  to  equalize  the  electrostatic  capacities  to  ground  of 
the  several  phases.  If  there  are  single-phase  branches  making  the  total 
lengths  of  the  three  conductors  unequal,  this  will  introduce  inequalities 
among  the  capacities  to  ground  which  it  may  not  be  possible  to  balance 
by  transpositions.  Inequalities  in  capacity  or  leakage  result  in  unequal 
voltages  between  the  different  line  conductors  and  ground. 

(6)  The  effect  of  unbalanced  loads  on  the  residual  voltage  is  very 
slight. 

(c)  With  balanced  loads  a  small  residual  current  consisting  of  un- 
balanced charging  current  may  flow  due  to  non-uniform  distribution  of 
unbalanced  capacity  and  leakage. 

(d)  Unbalanced  loads  have  but  a  slight  effect  upon  the  residual 
current. 

(e)  The  transformers  can  not  introduce  the  third  harmonic  and  its 
odd  multiples  as  residual  voltages  or  currents. 

NOTE. — Due  to  unsymmetrical  three-phase  connections  sometimes  employed  (such 
as  open-delta  and  Scott  connections)  the  third  harmonic  and  its  odd  multiples  may 
appear  in  the  voltages  between  lines  and  in  the  line  currents,  creating  dissimilarities 
in  the  wave  forms  for  the  several  phases.  These  harmonic  components  of  the  line 
voltages  and  currents  are  affected  by  unbalanced  capacity  and  leakage  in  the  same 
way  as  any  other  components  as  may  appear  in  the  residuals.  It  should  be  noted, 
however,  that  such  harmonics  are  not  impressed  directly  upon  the  line  as  residuals, 
as  in  the  case  with  grounded  neutral  systems. 

(/)  The  generators  can  not  introduce  the  third  harmonic  and  its  odd 
multiples  as  residual  voltages  and  currents. 

NOTE. — If  a  two-phase  generator  containing  a  third  harmonic  in  its  voltage  wave 
supplies  the  line  through  Scott  or  other  two  to  three-rjhase  transformer  connections 
the  third  harmonic  will  appear  in  the  voltage  between  lines.  Subject  to  the  conditions 
of  the  circuit  as  regards  capacity  and  leakage  balance,  this  harmonic  along  with  all 
others  may  or  may  not  appear  in  the  residuals. 


REPORT    ON    INDUCTIVE   INTERFERENCE.  27 

UNDER  ABNORMAL  CONDITIONS. 

(g)  A  ground  on  one  phase  causes  a  large  residual  voltage  (173 
per  cent  of  the  voltage  between  phases)  throughout  the  entire  length  of 
the  circuit.  A  residual  current  will  be  created  in  proximity  to  the 
fault,  its  magnitude  increasing  with  the  extent,  voltage  and  frequency 
of  the  system.  The  power  circuit  may  not  be  rendered  inoperative  and 
the  power  company  operators  may  be  unaware  of  the  existance  of  the 
abnormal  condition.  In  some  cases  the  residual  voltage  and  currents 
are  greatly  augmented  by  the  resonant  effects  accompanying  arcing 
grounds. 

(h)  An  open  condition  of  one  phase  may  cause  a  large  residual 
voltage,  a  certain  amount  of  residual  current  will  flow,  due  to  the  inter- 
change of  unbalanced  charging  current,  between  sections  of  line  on 
either  side  of  the  fault.  The  power  circuit  is  rendered  inoperative  for 
three-phase  supply  beyond  the  fault. 

A  consideration  of  the  characteristics  of  the  two  types  of  systems 
indicates  that  under  normal  operating  conditions  with  balanced  loads 
upon  all  phases,  the  residuals  of  the  grounded  neutral  system  may  be 
limited  to  the  third  harmonic  and  its  odd  multiples.  The  magnitude 
of  these  harmonics  is  dependent  largely  on  the  type  of  connection  on  the 
low  tension  side  of  the  transformer  banks,  the  delta  being  preferable 
to  star-connection.  Under  the  same  condition  the  residuals  of  the 
isolated  system  may  be  limited  to  those  resulting  from  unbalanced 
leakages  to  ground,  which  should  be  small  on  a  well  maintained  system. 
The  effect  of  an  unbalance  in  the  loads  connected  between  conductors 
upon  the  residuals  of  either  type  of  system  is  small,  while  the  effect  of 
an  unbalance  in  the  loads  connected  between  conductors  and  ground 
upon  a  grounded  neutral  system  is  to  cause  a  residual  current  which 
is  proportional  to  the  amount  of  such  unbalance  which  will  be  large  if 
the  unbalance  is  severe.  The  residual  current,  due  to  this  cause,  con- 
sists of  the  fundamental  and  all  harmonics  present  in  the  line  currents, 
in  addition  to  which  the  third  and  its  odd  multiples  are  introduced  as 
before  by  the  varying  permeability  of  the  transformer  iron,  and  in  some 
cases  by  the  generators. 

Under  abnormal  conditions  both  types  of  systems  give  rise  to  residuals 
which  are  liable  to  cause  interruption  and  damage  to  parallel  communi- 
cation circuits.  The  most  frequent  abnormal  condition  which  pro- 
duces severe  interference  is  an  accidental  ground.  A  ground  on  one 
phase  of  a  grounded  star-connected  system  creates  a  severe  and  wide- 
spread electromagnetic  unbalance,  giving  rise  to  corresponding  induc- 
tive effects.  This  is  accompanied  by  an  electrostatic  unbalance  in  the 
vicinity  of  such  ground.  On  the  lower  voltage  systems  this  latter  effect 
is  relatively  of  little  importance.  On  the  other  hand,  a  ground  on  one 
phase  of  an  isolated  system  creates  a  severe  and  widespread  electro- 
static unbalance,  giving  rise  to  corresponding  inductive  effects.  This  is 
accompanied  by  an  electromagnetic  unbalance  in  the  vicinity  of  the 
ground.  On  small  low-voltage  isolated  systems,  such  electromagnetic  un- 
balance is  relatively  of  little  consequence,  but  it  should  be  noted  that 
with  increased  voltage  and  extent  of  the  system  such  effects  do  become 
of  great  importance,  giving  rise  to  electromagnetic  disturbances  iri 
exposed  communication  circuits  in  addition  to  the  electrostatic  dis- 
turbances. 


28  REPORT    ON   INDUCTIVE   INTERFERENCE. 

The  magnitude  of  the  inductive  effects  from  either  type  of  system 
is  dependent  upon  the  character  of  the  exposure,  extent  of  the  power 
circuit  and  other  factors  which  render  it  impossible  with  the  information 
at  hand  to  draw  a  definite  conclusion  as  to  the  relative  total  amounts 
of  interference  inherent  with  the  two  types  of  system.  Furthermore, 
it  is  not  necessarily  true  that  either  type  of  connection  has  an  advantage 
from  the  inductive  interference  standpoint  for  power  systems  of  all 
sizes  and  voltages. 

3.     Means  for  Preventing  or   Reducing   Residual   Voltages  and  Currents. 

To  minimize  or  prevent  residual  voltages  and  currents  due  to  cause 
1,  it  is  necessary  to  equalize  as  closely  as  practicable  at  all  points  the 
load  between  the  several  phases  of  the  circuit  and  the  neutral,  or  to 
remove  the  ground  path  for  unbalanced  load  currents,  thus  allowing 
a  grounded  neutral  at  one  end  of  the  circuit  only.  As  it  is  difficult,  if 
not  impossible,  to  maintain  all  loads  in  a  state  of  equilibrium  at  all 
times,  the  latter  method  has  the  advantage  of  greater  reliability. 

Single-phase  connections  to  ground  should  not  be  employed.  Where 
single-phase  loads  or  unbalanced  three-phase  loads  must  be  supplied, 
the  transformers  supplying  such  loads  may  be  connected  across  the  line 
wires,  or  may  be  connected  star  to  delta,  with  the  neutral  not  grounded. 
It  should  be  noted  that  single-phase  or  unbalanced  three-phase  loads  on 
the  low  tension  or  delta  side  of  grounded  star  to  delta-connected  trans- 
formers produce  effects  on  the  high  tension  side  similar  qualitatively 
to  single-phase  loads  between  line  and  ground,  but  these  effects  are 
greatly  reduced  in  magnitude  by  the  inherent  balancing  influence  of 
transformers  so  connected,  due  to  the  fact  that  all  three  transformers 
participate  in  supplying  such  a  single-phase  load. 

Residuals  which  arise  from  cause  2  may  be  greatly  reduced  by  means 
of  certain  types  of  connections  for  generators  and  transformers.  Thus, 
for  example,  connecting  the  secondary  windings  of  the  transformer 
banks  in  delta  largely  suppresses  these  components  of  the  residual 
voltage  and  current  but  does  not  entirely  prevent  them.  Where  the 
transformers  are  connected  grounded  star  to  star,  these  components  can 
be,  to  a  certain  extent,  kept  out  of  the  line  by  the  use  of  a  second 
bank  of  transformers  having  a  delta  connection  on  one  side  and  a  star 
connection  on  the  side  in  common  with  the  first  bank  with  the  neutrals 
interconnected. 

The  possibility  of  the  introduction  of  third  harmonic  residuals  on  the 
line  due  to  the  use  of  grounded  star-connected  generators  may  be 
avoided  by  the  employment  of  transformers  between  generators  and 
line,  the  windings  on  the  generator  side  of  the  transformers  being 
isolated  from  ground. 

To  eliminate  or  reduce  residual  currents  and  voltages  which  may  be 
due  to  cause  3,  it  is  necessary  to  transpose  the  conductors  of  the  power 
circuit  so  as  to  equalize  the  electrostatic  capacities  of  the  several  phases 
to  ground,  and  this  equalization  must  be  attained  within  distances 
sufficiently  short  to  prevent  the  accumulation  of  large  unbalances.  With 
a  horizontal  arrangement  of  conductors,  the  capacities  to  ground  are 
more  nearly  equal  than  with  the  triangular  or  vertical  arrangement.  It 
is  probable  that  the  electrostatic  capacities  are  the  controlling  factors 
in  determining  the  residual  voltage  and  current  of  an  isolated  system 
under  normal  operation,  and  while  an  investigation  of  the  extent  to 


REPORT  ON  INDUCTIVE  INTERFERENCE.  29 

which  such  residuals  may  be  reduced  by  properly  spaced  transpositions 
has  not  as  yet  been  made,  it  is  reasonable  to  suppose  that  transpositions 
will  be  substantially  effective.  The  effect  of  unbalanced  leakage  can 
not  be  controlled,  except  through  proper  construction  and  maintenance 
of  the  power  system.  It  is  to  be  noted  that  the  maintenance  of  the 
system  free  from  accidental  grounds  and  partial  grounds  becomes  in- 
creasingly difficult  the  larger  the  extent  of  the  power  network. 

On  a  grounded  star-connected  system,  the  electrostatic  capacity  and 
the  leakage  of  the  several  phases  to  ground  are  relatively  less  effective  in 
producing  residual  voltage,  as  on  such  systems  the  voltages  to  ground 
are  determined  almost  entirely  by  the  generators  and  transformers. 

4.     Discussion   of  Tests. 

Having  given  a  general  analysis  of  the  causes  and  effects  of  and 
means  to  reduce  residual  currents  and  voltages,  it  is  desirable  to  call 
attention  to  the  results  of  tests  which  have  been  conducted,  which  have 
a  bearing  on  this  subject. 

At  Salinas  the  effect  of  grounding  or  isolating  the  neutral  of  the 
auto-transformers,  which  have  also  a  secondary  delta  winding,  was 
investigated.  These  auto-transformers  are  supplied  at  55,000  volts  over 
a  transmission  line  which  parallels  the  circuits  of  The  Pacific  Telephone 
and  Telegraph  Company  in  what  have  been  termed  exposures  No.  1 
and  No.  2.  These  auto-transformers  in  turn  supply  a  33,000-volt  line 
of  the  Coast  Valleys  Gas  and  Electric  Company,  extending  from  Salinas 
to  King  City,  a  distance  of  approximately  45  miles,  and  paralleling 
throughout  practically  this  entire  length,  the  coast.,  route  toll  lead  or 
The  Pacific  Telephone  and  Telegraph  Company.  These  same  telephone 
circuits  are  involved  in  the  parallels  with  the  55,000-volt  line  north  of 
Salinas.  In  addition  to  supplying  the  King  City  line,  this  bank  of 
auto-transformers  at  Salinas  supplies  a  22,000-volt  line  extending  to 
M<  nterey,  a  distance  of  approximately  18  miles.  Aside  from  the  ground 
on  the  transformer  neutral  at  Salinas,  there  are  no  grounds  on  either  the 
33,000-volt  line  or  the  22,000-volt  line.  The  55,000-volt  line  supplying 
the  Salinas  transformers  is  energized  at  the  Guadalupe  substation  of  the 
Sierra  and  San  Francisco  Power  Company,  approximately  73  miles  dis- 
tant from  Salinas  through  grounded  star-connected  auto-transformers, 
which  have  delta-connected  secondary  windings,  and  which  are  supplied 
by  the  104,000-volt  line  of  this  same  system  which  operates  with 
grounded  neutral  connections  at  its  main  generating  station  and  sub- 
stations. It  will  be  understood  from  this  statement  of  conditions  that 
the  neutral  current  at  Salinas  is  not  identical  with  the  residual  current 
of  any  one  of  the  three  high-tension  lines  which  are  connected  together 
by  these  auto-transformers.  The  condition  of  the  Salinas  neutral  affects 
the  induction  arising  from  the  several  exposures  through  its  effect  on  the 
residual  currents  and  voltages  of  the  high  tension  lines  connected  to 
the  auto-transformers  at  that  point.  A  representative  value  of  the 
neutral  current  at  Salinas  during  these  tests  is  0.3  ampere.  It  is 
compcsed  almost  entirely  of  the  ninth  harmonic,  the  fundamental  and 
the  third  harmonic,  their  magnitudes  decreasing  in  the  order  named. 
With  the  power  system  in  normal  operation,  isolating  the  neutral  of  the 
auto-transformers  at  Salinas  did  not  greatly  affect  the  resultant  induc- 
tion in  the  particular  exposures  under  observation.  The  values  in  the 
following  table,  taken  from  the  data  of  the  tests,  indicate  the  effect  of 


30 


REPORT    ON    INDUCTIVE    INTERFERENCE. 


the  condition  of  this  neutral  on  the  residual  currents  of  the  55,000-volt 
and  the  33,000-volt  lines. 

Residual  Current  at  Salinas — Amperes. 


53,000-volt  line 

33,000-volt  line 

Order  of 
harmonic 

Neutral  at  Salinas 

Neutral  at  Salinas 

Grounded 

Non-  grounded 

Grounded 

Non-grounded 

1 

0.120 

0.057 

0.061 

0.073 

3 

0.054 

0.160 

0.075 

0.120 

9 

0.073 

0.100 

0.120 

0.075 

Two  reasons  may  be  given  for  the  fact  that  the  condition  of  the 
Salinas  neutral  does  not  greatly  affect  the  resultant  residual  current  of 
these  lines :  (1)  The  load  balance  on  these  lines  is  such  that  a  relatively 
small  amount  of  load  current  flows  through  this  neutral;  (2)  As  three 
high  tension  lines  are  connected  together  by  these  auto-transformers, 
opening  their  neutral  connection  to  ground  does  not  completely  elim- 
inate the  path  for  the  residual  current  of  any  one  of  the  three  lines, 
since  it  may  then  flow  to  earth  through  the  admittance  to  ground  of 
the  other  two  lines. 

These  particular  conditions  are  not  commonly  found  but  a  similar 
condition,  in  that  there  is  a  path  to  ground  for  residual  current  aside 
from  the  neutral  connection,  prevails  in  any  case  where  the  power 
circuit  extends  for  a  considerable  distance  beyond  such  neutral  con- 
nection. The  investigation  showed,  for  the  conditions  which  applied  to 
the  55,000-volt  line,  that  removing  the  neutral  ground  connection  be- 
ycnd  the  parallel  decreased  the  fundamental  and  increased  the  third 
and  ninth  harmonics  in  the  residual  current,  as  shown  in  the  above 
table.  It  is  not  to  be  concluded,  however,  from  this  one  case  that  the 
third  harmonic  and  its  odd  multiples  in  the  residual  current  would  in 
all  cases  be  increased  by  removing  the  neutral  ground  connection  of  a 
bank  of  receiving  transformers  where  the  circuit  extends  beyond  the 
point  of  measurement  of  such  residual  current.  If  the  circuit  is 
terminated  at  the  transformer  bank,  the  removal  of  the  neutral  ground 
connection  must  eliminate  the  residual  current  at  that  point. 

In  the  case  of  the  33,000-volt  line,  the  grounding  of  the  neutral  at 
Salinas  merely  gave  another  and  nearer  grounded  neutral  point  on  the 
line  supplying  power,  but  did  not  give,  a  grounded  neutral  point  in 
each  direction  from  the  point  of  measurement  of  the  residuals,  as  it 
did  in  the  case  of  the  55,000-volt  line.  As  the  33,000-volt  line  has  no 
ground  connection  beyond  Salinas,  the  residual  current  must  flow  to 
ground  entirely  through  the  admittance  of  this  line  to  ground.  The 
residual  current,  therefore,  diminishes  to  zero  at  the  King  City  end  of 
the  line.  Isolating  the  neutral  of  the  Salinas  transformers  affects  the 
constituents  of  the  residual  currents  in  this  line  arising  from  the  Salinas 
transformers  and  those  impressed  by  the  55,000-volt  line,  in  such  a 
way  that  they  combine  vectorially  to  give  a  different  resultant  from 
that  with  the  Salinas  neutral  grounded.  The  result  is  to  increase  the 
fundamental  and  third  harmonic  and  to  decrease  the  ninth  harmonic 
when  the  neutral  is  isolated.  The  residual  current  in  the  22,000-volt 
line  was  not  determined,  but  residual  voltage  measurements  were  made 
with  the  Salinas  neutral  isolated  and  grounded  and  the  results  are  in- 


REPORT    ON    INDUCTIVE    INTERFERENCE. 


31 


chicled  in  the  following  table,  from  which  it  may  be  noted  that  the 
fundamental,  third  and  ninth  harmonics  were  all  greater  with  the 
Salinas  neutral  isolated. 

The  banks  of  star-connected  auto-transformers  at  the  Guadalupe 
and  Salinas  substations  are  provided  with  closed-delta  secondary  wind- 
ings, which  in  the  case  of  Salinas  supply  power  for  local  consumption. 
An  experimental  opening  of  the  delta  at  Salinas  demonstrated,  as  would 
be  anticipated,  that  the  use  of  such  delta-connected  secondary  windings 
reduces,  in  a  large  measure,  the  third  harmonic  introduced  by  these 
transformers  in  comparison  with  its  value  without  the  use  of  such 
delta-connected  windings.  If  grounded  star-connected  transformers 
are  used,  it  is  important,  therefore,  from  the  standpoint  of  induction, 
to  provide  such  transformers  with  closed-delta *  connected  secondary 
windings  or  with  other  means  of  reducing  the  third  harmonic  and  its 
odd  multiples.  Such  means  may,  however,  in  some  cases  be  insufficient 
to  reduce  the  residuals  to  such  low  values  that  they  will  not  produce 
harmful  inductive  interference  to  parallel  communication  circuits. 

The  investigation  on  the  system  of  the  Coast  Counties  Gas  and 
Electric  Company  shows  results  which  are  summarized  in  the  following 
table  with  reference  to  the  residual  current  and  residual  voltage.  Santa 
Cruz,  where  the  measurements  were  made,  is  20  miles  from  one  source 
of  supply  and  75  miles  from  the  other  end  of  the  line  where  power  was 
also  supplied.  For  the  sake  of  comparison  the  averages  of  the  residual 
voltage  of  the  22,000-volt  line  between  Salinas  and  Monterey,  a  distance 
of  18  miles,  are  also  given : 


Residual  voltage  —  volts 

Residual 

Salinas 

amperes 

harmonic 

Santa  Cruz 

Neutral 

Grounded 

Non-grounded 

i 

360 

50 

90 

0.094 

3 

— 

150 

320 

— 

9 

19 

40 

50 

0.021 

11 

14 

— 

— 

0.017 

13 

10 

— 

— 

— 

23 

14 

— 

•••• 

— 

The  system  of  the  Coast  Counties  Gas  and  Electric  Company  is 
isolated  from  ground  and  employs  a  number  of  Scott-connected  and 
open  delta-connected  transformers.  The  residuals  at  Santa  Cruz  on 
this  system  are  composed  principally  of  fundamental,  ninth  and 
eleventh  harmonics.  The  fundamental  is  predominant.  The  third 
harmonic  is  absent  or  too  small  to  measure  accurately.  It  should  be 
noted  here  that  the  use  of  Scott  and  open  delta-connected  transformers 
permits  the  third  harmonic  and  its  odd  multiples  to  exist  in  the  line 
voltages  and  currents  of  a  three-phase  isolated  system.  In  all  prob- 
ability the  residuals  on  this  system  are  caused  by  unbalanced  admit- 
tances to  ground  of  the  power  line  conductors.  As  has  already  been 
pointed  out,  that  part  of  the  unbalance  due  to  electrostatic  capacity 
could  be  greatly  reduced  by  properly  spaced  transpositions  in  the  power 
circuit.  In  contrast  to  the  results  at  Salinas,  the  residuals  of  this  sys- 
tem exhibit  a  prominent  fundamental  and  the  absence  of,  or  relatively 
small  amounts  of,  the  third  harmonic  and  its  odd  multiples. 


32  REPORT    ON   INDUCTIVE   INTERFERENCE. 

APPENDIX  III. 

TRANSPOSITIONS. 

The  sources  of  the  disturbances  in  communication  circuits,  which 
arise  from  parallel  power  circuits,  have  been  treated  in  the  first  section 
of  the  preceding  appendix.  The  effect  of  transpositions  on  the  induc- 
tion in  communication  circuits  produced  by  parallel  power  circuits  will 
now  be  considered. 

This  appendix  comprises  the  four  following  sections : 

1.  Effect  of  Transpositions  in  Reducing  Induction. 

2.  Characteristics  of  Present  Transposition  Systems. 

3.  Characteristics  of  Proposed  Transposition  Schemes. 

4.  Results  of  Tests. 

1.     Effect   of  Transpositions   in    Reducing    Induction. 

Transposing  a  circuit  is  the  interchanging  of  the  positions  occupied 
by  the  conductors. 

By  transposing  a  power  line  the  phase  of  the  resultant  electromag- 
netic field,  due  to  balanced  currents  and  the  phase  of  the  resultant 
electrostatic  field  due  to  balanced  voltages  is  changed,  and  the  induction 
is  reduced  by  the  production  of  neutralizing  effects  in  the  neighboring 
lengths  of  a  parallel  conductor.  Thus,  by  locating  the  power  circuit 
transpositions  so  that  each  conductor  occupies  all  of  the  several  possible 
conductor  positions  for  equal  distances,  a  section  or  "barrel"  is  obtained 
within  which  the  resultant  induction  on  a  parallel  conductor  due  to 
balanced  currents  and  voltages  is  completely  neutralized,  neglecting 
attenuation  and  remanent  electrostatic  effect  and  assuming  the  parallel 
is  uniform  throughout  the  barrel. 

Inasmuch  as  residual  currents  and  voltages  are  in  phase  in  the  sev- 
eral conductors,  the  transposition  of  the  power  circuit  does  not  reduce 
the  inductive  effects  therefrom  in  a  parallel  conductor,  except  as  the 
magnitudes  of  the  residual  currents  and  voltages  are  reduced  by  the 
power  circuit  transposition.  (See  Appendix  II.) 

As  usually  constructed,  the  conductors  of  a  telephone  circuit  are 
close  together  as  compared  with  their  distances  to  a  power  line,  and 
the  circuit  is  usually  isolated  from  ground.  Could  the  conductors  of  a 
metallic  communication  circuit  be  located  at  the  same  point  in  space,  as 
is  approximately  true  of  a  pair  of  wires  twisted  together,  the  resultant 
electromagnetic  and  electrostatic  induction  between  the  sides  of  the 
communication  circuit  would  be  zero.  The  voltage  induced  along  the 
conductors  of  the  telephone  circuit  and  the  induced  voltage  to  ground 
would  be  present  but  would  not  be  effective  in  producing  any  voltage 
between  the  conductors  of  the  telephone  circuit,  provided  the  capacity 
and  leakage  to  ground  of  each  side  of  the  telephone  circuit  were  equal. 
On  overhead  lines  the  conductors  of  a  metallic  communication  circuit 
must  be  at  least  several  inches  apart,  hence  in  general  when  paralleled 
by  a  power  line,  the  resultant  electromagnetic  and  electrostatic  induc- 
tion in  the  two  conductors  will  be  unequal  in  magnitude.  The  result 
is  that  a  voltage  exists  between  the  sides  of  the  circuit  which  causes  a 
current  to  flow  in  apparatus  connected  between  the  conductors,  such  as  a 
telephone  receiver. 

Transpositions  in  communication  circuits  tend  to  equalize  the  induc- 
tion in  the  two  sides  of  the  circuits  by  exposing  each  side  equally  to  the 


REPORT    ON   INDUCTIVE    INTERFERENCE.  33 

influence  of  the  power  circuit,  that  is,  by  reversing  in  successive  lengths 
the  phase  of  the  induction  between  the  two  sides  of  the  circuit. 

In  an  exposure  where  the  induction  from  balanced  currents  and 
voltages  would  be  completely  neutralized  by  the  power  circuit  trans- 
position system  if  there  were  no  communication  circuit  transpositions,  or 
where  such  induction  would  be  completely  equalized  by  the  communica- 
tion circuit  transpositions,  if  there  were  no  power  circuit  transpositions 
this  induction  will  practically  always  be  partially  cumulative  if  both 
power  and  communication  circuit  transpositions  are  installed  without 
due  reference  to  each  other.  It  should  be  noted,  however,  that  the  maxi- 
mum disturbances  which  may  be  set  up  in  a  parallel  communication 
circuit  by  balanced  currents  and  voltages  in  the  power  circuit  will  be 
present  when  neither  the  power  circuit  nor  the  communication  circuit 
is  transposed.  Hence  it  is  very  important  that  the  power  and  com- 
munication circuit  transpositions  be  properly  located  with  respect  to 
each  other,  and  in  this  way  only  can  the  maximum  benefits  from  the 
transpositions  be  derived. 

Induction  from  residual  currents  and  voltages  is  reduced  by  com- 
munication circuit  transpositions. 

If  the  communication  circuit  has  a  ground  return,  it  can  be  trans- 
posed and  the  power  circuit  transpositions  alone  will  be  effective  in 
reducing  interference  arising  from  the  balanced  currents  and  voltages. 
Also,  the  induction  into  a  ground  return  communication  circuit  from 
residual  currents  and  voltages  is  not  affected  by  transpositions,  except 
indirectly  as  previously  stated.  It  is  possible,  though  not  of  general 
practical  application,  to  obtain  the  effect  of  a  transposition  in  a 
grounded  alternating  current  power  or  communication  circuit  by  means 
of  a  transformer  or  repeating  coil. 

Induction  between  wires  and  ground  is  harmful  to  metallic  as  well  as 
to  ground  return  circuits,  for  in  case  the  metallic  circuit  is  not  perfectly 
balanced  electrically,  such  induced  voltage  forces  a  current  to  circulate 
in  the  metallic  circuit  through  the  terminal  apparatus.  It  is  not  prac- 
tical to  maintain  communication  circuits  in  a  state  of  perfect  balance  at 
all  times. 

2.     Characteristics  of   Present  Transposition   Systems. 

The  transposition  systems  used  on  long  distance  metallic  telephone 
circuits  are  designed  primarily  to  reduce  the  "cross-talk"  or  induction 
from  one  telephone  circuit  into  another,  and  provide  for  a  high  degree 
of  balance  between  any  circuit  and  all  others  on  the  line. 

The  length  of  standard  balanced  telephone  transposition  sections  used 
by  The  Pacific  Telephone  and  Telegraph  Company  is  approximately 
eight  miles  (more  exactly,  41,600  feet)  and  this  is  representative  of 
the  length  of  sections  of  the  transposition  systems  used  by  other  com- 
panies operating  similar  lines.  To  improve  the  transmitting  qualities  of 
telephone  circuits  used  for  long  distance  work,  loading  coils  are  intro- 
duced in  certain  circuits  at  the  ends  of  the  standard  transposition  sec- 
tions. Uniform  spacing  of  the  telephone  "S"  poles  (end  poles  of  trans- 
position sections)  is  an  important  consideration  in  the  application  of 
loading.  It  is  important  that  the  induction  be  neutralized  in  each  sec- 
tion between  loading  points,  as  these  are  points  of  discontinuity  in  the 
circuits. 


34  REPORT    ON    INDUCTIVE    INTERFERENCE. 

The  system  now  used  also  provides  for  the  transposition  of  every 
circuit  at  actual  intervals  ranging  from  one  quarter  mile  to  two  miles, 
the  average  intervals  for  different  circuits  varying  from  approximately 
one  quarter  mile  to  three  quarters  of  a  mile,  hence  every  circuit  is  to  a 
certain  extent  balanced  to  induction  from  parallel  power  circuits. 

In  addition  to  the  metallic  circuits  composed  of  two  conductors,  the 
telephone  companies  employ  phantom  circuits  which  are  made  up  from 
two  physical  (two  wire)  circuits.  Each  "conductor,"  or  side  of  the 
phantom  circuit,  consists  of  the  two  conductors  which  form  one  physical 
circuit.  As  usually  made  up,  the  physical  circuits  occupying  adjacent 
horizontal  positions  are  used  for  the  phantom  circuit.  Hence,  the 
average  distance  between  the  sides  of  the  phantom  circuit  is  equal  to 
twice  the  distance  between  the  conductors  of  the  physical  circuits.  Due 
to  the  greater  distance  between  the  sides  of  the  phantom  circuit  as  com- 
pared with  the  physical  circuits,  the  phantom  circuits  are  more  subject 
to  inductive  interference  than  the  physical  circuits.  The  phantom 
circuit  possesses  marked  advantages  in  economy  and  transmission 
efficiency  over  the  physical  circuits  composing  it,  hence  is  extensively 
used  for  the  longer  distances.  The  transpositions  in  the  phantom  cir- 
cuits are  spaced  at  average  intervals  for  different  circuits,  varying 
approximately  from  three  quarters  of  a  mile  to  two  miles. 

The  purpose  of  transposition  systems  applied  to  power  circuits  has 
been  to  reduce  the  disturbance  in  parallel  communication  circuits  and 
in  some  cases  to  equalize  the  separation  of  the  pairs  of  conductors  form- 
ing the  several  phases.  Usually  when  transpositions  have  been  applied 
to  power  circuits  to  reduce  the  disturbance  in  existing  parallel  communi- 
cation circuits,  one  or  more  complete  barrels  have  been  provided  within 
the  total  length  of  the  exposure.  The  best  obtainable  results  from 
power  circuit  transpositions  will  be  had  only  when  they  are  located 
with  due  regard  to  the  transposition  points  of  the  communication  cir- 
cuit. No  such  practice  as  this  has  been  followed  in  the  past.  The 
transposition  systems  heretofore  applied  to  parallel  power  and  com- 
munication circuits  have  therefore  failed  to  meet  the  requirements  for 
maximum  effectiveness.  Hence,  balanced  currents  and  voltages  in  the 
power  circuits  have,  in  general,  caused  more  disturbance  than  necessary 
in  parallel  communication  circuits. 

3.     Characteristics  of  Proposed  Transposition  Schemes. 

It  would  be  possible  to  fulfill  the  conditions  for  balance  with  regard 
to  induction  arising  from  balanced  currents  and  voltages,  by  cutting  a 
"barrel"  into  the  power  circuit  between  successive  communication  cir- 
cuit transpositions.  Inasmuch  as  telephone  transposition  points  are 
ordinarily  spaced  at  one  fourth  mile  intervals,  this  solution  in  the  case 
of  a  three-phase  power  circuit  would  necessitate  transpositions  at  an 
average  spacing  of  one  eighth  mile  and  a  minimum  spacing  of  one 
twelfth  mile,  which  is  impracticable  in  most  cases. 

It  would  be  po  sible  to  satisfy  the  conditions  for  balancing  the  induc- 
tion in  metallic  circuits,  from  both  balanced  and  residual  currents  and 
voltages,  by  installing  any  completely  balanced  system  of  communication 
circuit  transpositions  between  each  two  successive  power  circuit  trans- 
positions. Assuming  twelve  mile  "barrels"  in  the  power  circuit,  the 
conditions  for  balance  could  be  fulfilled  with  the  present  standard  tele- 
phone transposition  system.  However,  with  power  circuit  barrels  of  a 


REPORT    ON   INDUCTIVE   INTERFERENCE.  35 

length  such  as  is  essential  in  most  parallels,  this  solution  would  require 
the  redesign  and  relocation  of  all  telephone  transpositions  in  the  ex- 
posure, involving  several  times  as  many  transpositions  as  are  nor- 
mally required,  with  the  liability  of  interference  with  the  location  of 
loading  coils. 

Both  the  above  solutions  satisfy  the  conditions  for  balancing  the  induc- 
tion in  metallic  circuits,  arising  from  residuals,  in  length  of  circuit 
equal  to  or  twice  the  distance  between  successive  communication  circuit 
transpositions,  assuming  these  are  uniformly  spaced.  In  the  standard 
transposition  section  as  now  used,  balance  is  thus  obtained  in  distances 
varying  from  an  average  of  approximately  one  fourth  to  four  miles. 

Between  these  two  comparatively  simple  but  extreme  solutions  the 
practical  but  more  complicated  solution  for  general  cases  is  to  be 
obtained.  This  involves  the  combination  of  power  circuit  "barrels"  of 
moderate  length  with  a  modified  communication  circuit  transposition 
system  designed  to  procure  balance  as  far  as  practicable  for  all  circuits. 
In  this  way  co-ordinated  transposition  systems  may  be  designed  which 
are  sufficiently  flexible  to  meet  the  requirements  of  short  parallels  and 
portions  of  longer  parallels  separated  by  points  of  discontinuity. 

In  the  discussion  above  with  reference  to  schemes  of  transpositions 
the  balances  or  unbalances  mentioned  are  those  which  would  occur,  due 
solely  to  the  relative  locations  of  transpositions  in  an  exposure  whose 
physical  characteristics  are  uniform  throughout.  Even  with  a  scheme 
of  transpositions,  balanced  in  the  sense  described,  applied  to  both  power 
and  communication-  circuits  involved  in  an  actual  parallel,  there  are  a 
number  of  factors  as  noted  below,  which  in  general  are  not  capable  of 
being  taken  into  account  quantitatively  and  because  of  which  effective 
neutralization  may  not  be  obtained.  These  factors  are: 

1.  Non-uniformity  of  separation,  configuration  and  other  physical 
characteristics. 

2.  Variation  in  magnitude  and  phase  of  the  inductive  effects 
along    the    exposure    (applying    particularly    to    the    higher    fre- 
quencies). 

3.  Inherent  inability  of  transpositions  to  completely  neutralize 
electrostatic  induction  (this  remanent  effect  can  be  reduced  as  far 
as  desired  by  inserting  a  sufficient  number  of  transpositions). 

4.  Imperfect  electrical  balance  of  the  communication  circuit. 

While  these  factors  which  prevent  complete  neutralization  of  the  in- 
duction can  not  be  entirely  eliminated,  their  effects  can  be  abated  by 
reducing  the  length  of  balanced  transposition  sections.  Thus  it  is  not 
sufficient  merely  to  install  transpositions  in  both  lines  so  that  they  are 
balanced  to  each  other ;  but,  also,  it  is  necessary  to  take  into  considera- 
tion the  length  of  section  within  which  balance  is  obtained  and  to  make 
this  length  as  short  as  the  conditions  of  the  particular  case  require. 

Points  of  discontinuity,  such  as  abrupt  changes  in  power  line  cur- 
rent where  a  material  amount  of  load  is  taken  off,  cross-overs,  or  sub- 
stantial changes  in  separation,  should,  if  practicable,  be  made  neutral 
points  (junction  points  of  balanced  sections)  in  the  transposition 
scheme.  Where  cross-overs  occur  balance  should  in  general  be  obtained 
independently  for  the  portions  of  the  communication  line  on  each  side 
of  "the  power  circuit. 


36  REPORT  ON  INDUCTIVE  INTERFERENCE. 

The  transposition  system  and  the  location  and  spacing  of  transposi- 
tion poles  are  factors  of  prime  importance  in  the  successful  operation 
of  telephone  lines,  on  account  of  the  mutual  effects  among  the  many 
circuits  carried  on  such  lines.  On  the  other  hand,  transpositions  in 
power  circuits  are,  relatively,  of  minor  importance  in  the  operation  of 
a  power  system  and  from  this  standpoint  the  effect  of  small  changes  in 
the  location  of  such  transpositions  is  negligible.  Hence,  in  general,  the 
requirements  of  the  communication  circuits  are  the  chief  factors  which 
should  govern  the  location  of  all  transpositions  in  both  power  and  com- 
munication circuits. 

An  individual  study  is  necessary  to  determine  the  best  procedure  for 
any  given  parallel  owing  to  the  wide  variation  in  conditions.  Thus  only 
is  it  possible  in  each  case  to  determine  the  best  location  and  method  of 
transpositions  with  regard  to  the  requirements  of  both  power  and  com- 
munication systems. 

4.     Results   of  Tests. 

The  investigation  at  Salinas  demonstrated  that  the  induction  in  a 
ground  return  circuit  in  the  exposures  concerned  arises  principally 
from  the  residual  voltages  and  currents,  while  the  induction  in  a  metallic 
circuit  shows  principally  the  characteristics  of  the  balanced  voltages  and 
currents  together  with  some  effect  from  the  residuals.  This  result  was 
to  be  expected  as  there  are  power  circuit  transpositions  which  reduce 
the  induction  in  the  conductors  used  as  ground  return  circuits,  due  to 
the  balanced  components,  but  these  transpositions  and  the  transpositions 
in  the  telephone  circuits  are  improperly  located  with  respect  to  each 
other  and  therefore  are  inefficient  as  regards  the  induction  in  the 
metallic  circuits.  On  the  other  hand,  the  telephone  transposition  system 
tends  inherently  to  reduce  the  induction  in  the  metallic  circuits,  arising 
from  residuals.  A  study  of  the  relative  location  of  power  and  tele- 
phone circuit  transpositions  for  exposure  No.  2  at  Salinas,  indicated  that 
by  modifying  the  present  transpositions  of  both  circuits,  it  is  possible 
to  reduce  materially  the  induction  from  balanced  currents  and  voltages. 
Had  it  been  feasible  to  take  the  power  circuit  out  of  service  for  the 
purpose  of  experimental  retransposition,  the  above  scheme  as  well  as 
one  for  the  King  City  exposure,  would  probably  have  been  installed  and 
the  effects  thereof  experimentally  determined.  Under  the  conditions 
existing,  however,  it  was  deemed  advisable  to  postpone  the  matter  of 
transpositions  for  both  these  exposures,  pending  the  acquisition  of 
further  information  as  to  the  extent  to  which  retransposition  would  be 
warranted  as  a  permanent  improvement. 

The  experimental  study  of  transpositions  was,  therefore,  transferred 
to  another  point  where  a  power  line  is  not  the  sole  source  of  supply  and 
can,  therefore,  be  shut  down  for  alterations  and  tests  under  special 
conditions. 

The  experimental  determination  of  the  practical  effectiveness  of 
transpositions  has  not  been  completed.  However,  an  extended  theo- 
retical study  of  transpositions  has  been  made,  including  the  design  of  a 
modified  telephone  transposition  system.  This  system,  which  requires 
many  additional  transpositions,  is  more  flexible  in  its  properties  of  co- 
ordination with  different  lengths  of  power  circuit  "barrels." 

A  study  made  to  determine  the  relative  efficiency  of  various  schemes 
of  transpositions  designed  for  the  Santa  Cruz-Watsonville  exposure  of 


REPORT    ON    INDUCTIVE   INTERFERENCE.  37 

The  Pacific  Telephone  and  Telegraph  Company's  toll  lead  to  the  22,000- 
volt  line  of  the  Coast  Counties  Gas  and  Electric  Company,  emphasizes 
the  following  general  principles: 

1.  The  necessity  of  proper  relative  location  of  power  and  tele- 
phone circuit  transpositions. 

2.  The  importance  of  the  effect  of  cross-overs  and  the  desirability 
of  making  them  neutral  points  in  the  transposition  scheme. 

3.  The  necessity  of  some  modifications  of  the  telephone  trans- 
position system. 


38  REPORT    ON   INDUCTIVE   INTERFERENCE. 

APPENDIX  IV. 

APPARATUS. 

For  the  proper  conduct  of  its  tests  and  experiments  the  Joint  Com- 
mittee on  Inductive  Interference  has  secured,  either  through  purchase 
or  on  loan  account  from  various  power  and  communication  interests, 
apparatus  of  an  aggregate  value  of  over  twelve  thousand  dollars. 

The  following  is  a  brief  schedule  of  the  property  in  use  by  this  com- 
mittee, together  with  its  estimated  replacement  value : 

Buildings    (portable   laboratory)    $480  00 

Furniture  and  fixtures 128  00 

Apparatus — • 

Oscillograph $1,11500 

Oscillator    600  00 

Motor  generator  set 260  00 

Meters    1,202  50 

Batteries    100  00 

Condensers    990  00 

Bridges 675  00 

Galvanometers    265  00 

Rheostats 7#4  80 

Switchboards    135  40 

Miscellaneous    apparatus    1,505  00 

Coils  and  relays 645  00 

Transformers    2.412  50 

Miscellaneous    787  00 

Photographic 293  60        11,820  SO 


Grand   total   $12,428  80 

The  above  property  is  owned  by  the  Joint  Committee  on  Inductive 
Interference  and  various  power  aad  communication  companies  as 
follows : 

Joint   Committee   on    Inductive   Interference $1,251  15 

The    Pacific    Telephone    and    Telegraph    Company    and    American 

Telephone  and  Telegraph  Company 8,293  65 

Sierra  and  San  Francisco  Power  Company __  2,002  50 

San  Joaquin  Light  and  Power  Company 300  00 

Pacific  Gas  and  Electric  Company 110  00 

Western  Union  Telegraph  Company 235  00 

Testing    force—  256  50 


Total $12,428  80 


REPORT    ON    INDUCTIVE    INTERFERENCE. 


39 


APPENDIX  V. 

LIST  OF  TECHNICAL    REPORTS. 

The  following  is  a  list  of  the  technical  reports  which  have  been  pre- 
pared in  connection  with  the  investigation  of  the  Joint  Committee  on 
Inductive-  Interference : 


Subject 


General  outline  of  tests  to  be  made  at  Salinas  on  parallels  between  lines  of  the  Sierra 

and   San  Francisco   Power  Company,   the  Western  Union  Telegraph   Company,   the 

Southern    Pacific    Company,    and   The   Pacific   Telephone    and   Telegraph    Company. 

(6  pages.) 
Summary  of  results  of  tests  at  Morgan  Hill  on  parallel  between  lines  of  the'  Coast 

Counties  Gas  and  Electric  Company  and  The  Pacific  Telephone  and  Telegraph  Com- 
pany between  Morgan  Hill  and  Gilroy.     (8  pages.) 
A  description  of  the  noise  standard  in  use  for  measuring  noise  on  telephone  circuits  in 

terms  of  a  standard  unit.    (4  pages.) 
A  description  of  the  instruments  and  methods  used  for  the  measurement  of  effective 

values  of  induced  voltages  and  currents.     (2  pages.) 

A  description  of  apparatus  and  connections  used  in  measuring  line  and  residual  cur- 
rents and  voltages  of  power  circuits.     (6  pages.) 
Tests  of  the  effects  of  opening  the  secondary  delta  of  the  auto-transformer  bank  at 

Salinas.     (7  pages.) 
Tests  of -the  induction  in  the  block  signalling  circuits  of  the  Southern  Pacific  Company 

paralleled  by  the  Salinas-King  City  circuit   of  the   Coast  Valleys  Gas   and  Electric 

Company.     (4  pages.) 
Tests  of  the  induction  in  the  telephone  circuits  of  exposure  No.  2  at  Salinas  under 

normal  operating  conditions   of  the  power  system  with  particular  reference  to  the 

effects   of   grounding    and   isolating   the  neutral   of   the  Salinas    auto-transformers. 

(16  pages.) 
Experimental  determination  of  the  coefficients  of  induction  for  residual  currents  and 

voltages  in  exposure  No.  2  at  Salinas.    (4  pages.) 

Measurements  of  the  harmonics  of  the  neutral  current  at  Salinas.    (4  pages.) 
Investigation   of  current   transformers,   ratios,    and  errors  due  to  the  use  of  current 

transformers  under  the  conditions  of  the  tests.     (21  pages.) 
Formulae    for   the   computation    of    electrostatic    and   electromagnetic   induction    from 

power  circuits  in  neighboring  communication  circuits.     (18  pages.) 
An   investigation  of  errors  in  measurements  of  residual  voltage  due  to  the  potential 

transformers    used    and    a    discussion    of   the   method    of   measurement    at    Salinas. 

(30  pages.). 
Comparative  tests  of  the  noise  in  exposed  telephone  circuits  with  power  on  and  off 

the    55,000-volt    power    circuit    of    the    Sierra    and    San    Francisco    Power    Company 

between  Guadalupe  and  Salinas.    (8  pages.) 
Supplementary  to  Technical  Report  No.  8,  differing  from  the  earlier  report  in  that  the 

telephone  circuits  were  shielded'.    Contains  a  discussion  of  transpositions.    (22  pages.) 
Tests  of  the  induction  in  telephone  circuits  exposed  to  the  Coast  Counties  Gas  and 

Electric  Company's  22,000-volt  line  between  Morgan  Hill  and  Gilroy  with  the  power 

circuit  untransposed  and  open  at  Gilroy.     (4  pages.) 
Tests  of  the  induction  in  telephone  circuits  exposed  to  the  Coast  Counties  Gas  and 

Electric  Company's  22,000-volt  line  between  Morgan  Hill  and  Gilroy,  before  and  after 

installing  power  circuit  transpositions.     (25  pages.) 
Tests  of  the  effect,  on  exposed  telephone  circuits,  of  grounding  one  phase  of  the  Coast 

Counties    Gas    and    Electric   Company's   22,000-volt   three-phase   delta-connected   line. 

(4  pages.) 
Tests  of  the  combined  effects  of  the  Coast  Counties  Gas  and  Electric  Company's  and 

the  Sierra  and  San  Francisco  Power  Company's  circuits  on  the  telephone  circuits  in 

the  exposure  between  Morgan  Hill  and  Gilroy.     (4  pages.) 
Tests  of  the  effect  on  the  residual  voltage  of  transposing  the  Coast  Counties  Gas  and 

Electric   Company's   22,000-volt  line  within  the  exposure  between   Morgan   Hill   and 

Gilroy.     (3   pages.) 
Tests  to  determine  the  comparative  effect  on  the  noise  in  the  exposed  telephone  circuits 

of   having  the  power  on  .and  off  the  Coast  Counties  Gas  and  Electric  Company's 

22,000-volt   1  etween   Morgan  Hill   and   Gilroy,    and   the  effect  of   shielding  the  tele- 
phone circuit  under  test  by  grounding  other  circuits  on  the  lead.    (4  pages.) 
Computation  of  the  coefficients  of  induction  from  balanced  and  residual  currents  and 

voltages  for  the  telephone  circuits  of  exposure  No.  2  at  Salinas.    (19  pages.) 
Experimental  determination  of  the  coefficients  of  induction  from  residual  currents  and 

voltages,  for  the  telephone  circuits  of  exposure  No.  2  at  Salinas— more  complete  than 

Technical  Report  No.  9.    (24  pages.) 


40 


REPORT    ON   INDUCTIVE    INTERFERENCE. 


Tech- 
nical 
report 
number 


Subject 


Comparison   of  computations   of   Technical  Report   No.   22  with  experimental  data   of 

Technical  Report   No.   23.     (16  pages.) 
Tests   of   induction   in   telephone   circuits   in  exposure   between   Salinas   and  King   City 

under  normal  operating  conditions,  with  the  neutral  of  the  Salinas  auto-transformers 

grounded  and  isolated.     (20  pages.) 
Tests  of  accuracy  of  measurement  of  residual  current  by  certain  current  transformers. 

(4  pages.) 
Tests  of  induction  in  telephone  circuits  in  exposure  No.   2  at  Salinas  with  the  North 

Beach  steam  station  energizing  the  Sierra  and  San  Francisco  Power  Company's  line. 

Supplementary  to  Technical  Reports  Nos.  8  and  15,  differing  in  the  source  of  supply 

of  the  power  system.     (27  pages.) 
Supplementary  to  Technical  Reports  Nos.  8  and  15.    Voltage  lowered  5  per  cent  at  the 

Guadalupe  auto-transformers  which  supply  the  power  circuit.     (20  pages.) 
Determination    of    impedances    of    lines,     by    computations    and    by    measurements — 

numerous  curve  sheets  and  tables.     (65  pages.) 
Tests  of  induction  in  telephone  circuits  in  exposures  Nos.  1  and  2  at  Salinas,  with  the 

neutral  of  the  Salinas  transformers  grounded  and  isolated.     (10  pages.) 
Supplementary  to  Technical  Reports  Nos.  8  and  15  and  more  complete.     Includes  tests 

with  Salinas  neutral  grounded  and  isolated  and  with  telephone  circuits  shielded  and 

unshielded.     (29  pages.) 

Supplementary  to  Technical  Report  No.  25.     (22  pages.) 
Induction  in  test  leads  used  at  Salinas  for  connecting  testing  apparatus  to  the  circuits 

of   exposure   No.  2     and  the  effect   of   such   on  the  measurements   of   the  induction 

from  the  exposure.     (20  pages). 
Effect  of  changes  in  the  insulation  resistance  of  the  telephone  line  on  the  induction  in 

telephone  circuits  of  exposure  No.  2  at  Salinas.    Also  supplements  Technical  Reports 

Nos.  8,  15,   and  31.     (24  pages.) 
General  outline  of  tests  to  be  made  at  Santa  Cruz  on  the  parallel  between  lines  of  the 

Coast  Counties  Gas  and  Electric  Company  and  The  Pacific  Telephone  and  Telegraph 

Company.     (4  pages.) 
Induction   in    telegraph   circuits    of   the   Western   Union   Telegraph    Company    and   the 

Southern  Pacific  Company  in  exposure  No.  1  between  Salinas  and  San  Jose.     (8  pages.) 
Noise    tests    on   telephone   circuits   radiating   from   Salinas,    with   the  neutral   of   the 

Salinas  auto-transformers  grounded  and  isolated.     (4  pages.) 
General  review  of  tests  at  Salinas,  summarizing  reports,  1,  6,  7,  8,  9,  10,  11,  12,  13,  14, 

15,  22,  23,  24,  25,  26,  27,  28,  30,  31,  32,  33,  34,  36,   and  37.     (53  pages.) 
General  consideration  of  transpositions  and  a  study  of  the  results  to  be  expected  from 

the    application    of    various    transposition    schemes    to    the    Santa    Cruz-Watsonville 

exposure.     (36  pages.) 

Method  of  measurement  of  capacity  and  conductance  unbalances.     (2  pages.) 
Harmonic  analysis  of  alternating  current  waves,  by  oscillograph  and  resonant  shunt. 

Comparison  of  the  methods.     (30  pages.) 
Investigation   of   the   current   transformers   in  use  at   Santa   Cruz,    to   determine  their 

ratios    of    transformation    and    suitability    for    residual    current    measurements.     (35 

pages.) 
Outline  of  tests  to  determine  the  effect  of  extraneous  currents  on  the  intelligibility  of 

telephone  conversation.    (8  pages.) 
Induction  in  the  telephone  circuits  of  the  Santa  Cruz-Watsonville  exposure  and  in  the 

test  leads,   from  sources  other  than  the  22,000-volt  line.     (12  pages.) 
Induction  in  the  telephone  circuits  of  the  Santa  Cruz-Watsonville  exposure  under  com- 
mercial  operating   conditions,    with   the   original   transpositions    in  both   power   and 

telephone  lines.     (15  pages.) 
Supplementary    to    Technical    Report    No.    39.      A    study    of    additional    transposition 

schemes  for  the  Santa  Cruz-Watsonville  exposure.     (14  pages.) 
Computation   of  the  coefficients   of  induction   for  balanced   and  residual  currents   and 

voltages  for  the  Santa  Cruz-Watsonville  exposures.     (11  pages.) 
Experimental  determination  of  coefficients  of  induction  in  the  Santa  Cruz-Watsonville 

exposure,   with  the  original  transpositions.     (42  pages.) 
Further  experimental  determination  of  coefficients  of  induction  for  balanced  voltages, 

in  the  Santa  Cruz-Watsonville  exposure,  with  the  original  transpositions.     (13  pages.) 
Study   of  the   influence   of   various  transformer  connections   and  flux  densities   on  the 

third  harmonic  and  its  multiples  in  a  three-phase  circuit.     (In  preparation.) 


REPORT   ON   INDUCTIVE   INTERFERENCE. 


41 


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INDEX 


PAGE. 

LETTER  OF  COMMISSION  TO  THE  JOINT  COMMITTEE 3 

LETTER  OF.TRANSMITTAL 4 

REPORT 

SCOPE ; 5 

HISTORICAL 5 

Formation  of  Committee 5 

Its   object    5 

Organization   5 

Changes  in  Membership 6 

Field  Engineering  Staff 6 

Parallels  Investigated 7 

Technical  Reports 7 

RESULTS  ACCOMPLISHED  8 

1.  Effect  of  Harmonics  on  Telephone  Circuits 8 

2.  Effect  of  Fundamental  on  Telephone  Circuits 8 

3.  Effect  of  Fundamental  on  Telegraph  Circuits 8 

4.  Balanced   and  Residual   Components 8 

;~».  Effect  of  Transpositions 8 

G.  Abnormal  Conditions 9 

RULES  RECOMMENDED  BY  THE  COMMITTEE 9 

DEFINITIONS 10 

I.  AVOIDANCE  OF  PARALLELISM 10 

II.  CONDITIONS  UNDER  WHICH  PARALLELISM  WILL  BE  PERMITTED 10 

III.  PROVISIONS  APPLYING  TO  EXISTING  PARALLELS 14 

IV.  WAIVER  OF  CONDITIONS  BY  COMMUNICATION  COMPANY 14 

V.  PARALLELISM  WITH  ALTERNATING  CURRENT  RAILWAYS 14 

DISCUSSION   OF   RECOMMENDATIONS 15-18 

FUTURE    WORK 18,19 

APPENDICES 20 

Appendix      I.  Harmonics    . 20,  21 

Appendix    II.  Balanced  and  Residual  Voltages  and  Currents 22-31 

Appendix  III.  Transpositions    32-37 

Appendix  IV.  Apparatus    38 

Appendix    V.  List  of  Technical  Reports 39,40 

Appendix  VI.  Organization  Chart 41 


